FOUR YEARS AGO, when Marita Mullan of Philadelphia, Pa., was 15 years old, a boy who knew she liked animals gave her a pair of common house mice. The joke backfired. Her growing enthusiasm for the new pets soon left little time for him. She started haunting the public library and registered for special courses at the University of Pennsylvania, though she was too young to receive credit. Within a little more than two years she was invited to address the scientific staff of the Jackson Memorial Laboratory in Bar Harbor, Me. With the help of her mice, Miss Mullan had become an advanced amateur geneticist.

Miss Mullan's interest in genetics was sparked by a chance observation. One morning, when she went to the basement to water her mice, she made a strange and, as she learned later, rare discovery. A few days earlier the mice had produced a litter of six young, all pink and hairless. Now they were growing coats. This morning she noticed that one of the baby mice was not a mousy gray, like its parents, but bright orange! In the course of searching for the answer she bred mice of many colors and learned why the average householder is unlikely ever to trap an orange one.

She began with a review of the work of genetics' most celebrated amateur: the Austrian monk Gregor Johann Mendel. Mendel also started with a question. What factor, he wondered, relates true-breeding varieties within a species? For the answer he put the question directly to nature. He had had no previous training in science and so was forced to discover its method for himself: he stated his problem, experimented, observed, theorized, validated his theory by further experiment and finally expressed his findings as a set of natural laws.

Although Mendel worked with peas his basic laws of heredity hold equally for mice, oak trees or men-for any organism that reproduces sexually. He cultivated and crossbred two varieties of peas-tall and dwarf. When the offspring of these matured, he was surprised to observe they had all grown tall, like one of the parents, instead of medium-sized as he had expected. He then interbred these tall hybrids. Again he was surprised. This time the offspring were both dwarf and tall-and in the precise ratio of three tall plants to one dwarf.

What set of circumstances, Mendel wondered, would produce the orderly result he had observed? After much pondering he finally hit upon a theory that goes like this: Suppose the cells of the parent plants each contain a pair of factors in the form, for instance, of minute particles. One kind of particle can cause a plant to grow tall and the other dwarf. Suppose further that when the male germ cell of one plant unites with the female germ cell of the other, each contributes only one member of its pair of particles to the seed. The factor from this male parent, let us say, is invariably of the type causing tallness, and from the female, dwarfism. Then all members of the first hybrid generation from this pair of parents would get a tallness factor from the "father" and a dwarfism from the "mother." Assume that the tallness factor is dominant. Then all the offspring would grow tall.

But when these hybrids interbred in turn, a different result would be expected. The inheritance could now be mixed in four different combinations: tallness plus tallness, tallness plus dwarfism, dwarfism plus tallness, dwarfism plus dwarfism. The first three of these combinations would produce a tall plant, the last a dwarf plant. So on the average three out of four of the second generation should be tall.

It was a clever explanation-but did it really describe nature? Mendel tested it statistically on various hereditary characteristics of peas-the shape and color of the pods, the position of the flowers, the length of the stems and the texture of the seeds-and found that in every experiment his theory accurately predicted the results.

Today Mendel's mysterious factors are called genes. They are considered "atoms" of heredity-minute bits which may be studied only indirectly, by their effects, because they are too small to be seen even under the electron microscope.

Every organism begins life as a single cell. The genes are found in the cell's nucleus, arranged in threads or filaments like strings of beads. The threads are called chromosomes, from the fact that they can be stained with colored dyes for observation under the microscope. When a cell prepares to divide, the barely visible chromosomes gradually thicken and finally split down their length, each of the many genes in each new piece being duplicated in the other. Thus each of the daughter cells inherits a duplicate set of chromosomes and genes.

In all of nature the gene is the only known structure with the power to manufacture an exact copy of itself. But this does not mean that a gene cannot be changed. Every now and again some force acts on one gene or another and alters it. X-rays will do this. So will cosmic rays, various chemicals, extremes of temperature and other influences. Unless the affected gene is destroyed in the process, it will subsequently go right on making copies of itself-in its new, altered form. Such modified genes are called mutants. They account not only for the varieties within the species but also provide the basis for the origin of new species through natural selection. Some mutations are beneficial to the organism's chances of survival, but most are harmful. Fortunately most of the desirable ones are dominant and hence assist the development of useful traits. In contrast, recessive genes carrying a potential of undesirable characteristics can express themselves only when chance happens to pair them with like recessives in an offspring. In a large population the chances of such a meeting are small. Hence many generations may come and go before the unfortunate trait appears.

Marita Mullan's orange mouse represented such a rare event. The gene responsible for the trait is known as a "lethal yellow." As the name implies, this gene carries other changes more serious than the orange color. Lethal yellow mice die before they are old enough to mate. Miss Mullan's orange mouse died within a few weeks.

Such lethal mutants appear in all species. Fox breeders, for example, cultivate a highly prized variety of "platinum" fox. When platinum foxes mate, the hybrid is a snowy white. Like orange mice they never survive. Several lethal genes are carried by man, and science has learned how to circumvent the effects of some. Diabetes is genetic in origin; it is due to a defect or absence of the gene responsible for the manufacture of insulin. This genetic failure must be offset by administering insulin artificially.

The treatment of diabetes provides a clue to the nature of the chemical mechanism through which genes express themselves. The genes apparently govern the complex chemistry of cells, each gene being responsible for the cell's ability to manufacture a particular chemical link. This means that a great number of different kinds of chemicals must bathe the center of the cell, because complex organisms such as mice and men exhibit thousands of different traits, each accounted for by its own unique gene and corresponding chemical substance. It follows also that in this conglomerate chemical stew all the genes must to some extent modify one another's effects. Few genes, if any, act in complete independence.

In view of the interaction of the genes, it is logical to suppose that their position on the chromosomes could play a major role in shaping inherited traits. This is indeed the case, and it accounts for another mechanism by means of which varieties arise within a species. Sometimes when the new cell makes its initial division, the chromosomes break and grow together again in a different order, or several may break and exchange sections during reassembly. Short lengths, together with their complement of genes, may even be lost in the process. Any of these and related chance happenings may result in an offspring which carries a genetic structure differing from that of the parents. Hence, the offspring may possess a set of characteristics strange to the lineage-some visible and others obscure.

After a time, particularly in large populations where individuals mate outside the family line, the most dominant traits emerge. All individuals within the species bear marked superficial resemblance to one another, although the genetic systems of the mating partners may carry hundreds, even thousands, of contrasting and hidden recessives. Many of the recessives may be endowed with potential control over some one trait, such as hair color, but may never express themselves until they become paired with others of like kind through chance mating. Thus variation in many traits is subject to the control of a whole group of genes.

It was with such a group that Marita Mullan worked. She writes:

"Little did I realize the storehouse of potential beauty that my original pair of mice were hiding in the form of recessive mutants. Gradually, however, many of these varieties appeared in the offspring. Most of them are well known. Some were described long before the time of Christ by the Chinese, who bred these animals because of their singular beauty. Several unlisted traits came to light during my experiments.

"The most treasured of all fancy genes, and the most beautiful, is the pink-eye dilution. This gene somewhat reduces the amount of pigment in the skin and eyes. It also tends to produce a smaller mouse. The maltese or blue dilution is another mutant that tends to reduce the amount of pigment, but less drastically than the pink-eye gene. Black and brown is another striking combination and produces a blue-grey coat and a very beautiful chocolate color.

"The basic colors of the mouse's fur are produced by what is called the agouti series. 'Agouti' means that the fur has a characteristic variegated appearance, caused by the fact that each hair has a light and dark portion. It accounts for the typical mousy appearance of the wild mouse's fur. The series also contains the light-bellied agouti, the black-and-tan and the so-called non-agouti or black."

The work of a geneticist differs from that of a person who simply breeds animals or plants. In the first place, the geneticist hopes to learn more about the mechanism of heredity and, if possible, about the scientific basis of life itself. In the second place, the geneticist's breeding technique is guided by tested and proved laws, while the breeder generally proceeds on the rule of thumb that "like produces like."

Breeding by this classical method has gradually improved the stock of many plants and animals. By selective mating of individuals exhibiting the desired traits, man has developed products ranging from rust-resistant wheat to race horses. But after a time this method reaches a limit: no amount of careful selection seems to make any additional improvement. The quality of the stock levels off.

Genetics can do much better. At the turn of the present century. Midwestern farmers were content with a strain of corn, for example, which yielded about 25 bushels per acre on the average. Today hybrid corn, developed by scientific application of principles of genetics produces yields exceeding 200 bushels per acre!

How does genetics achieve such sensational improvement? Primarily by close inbreeding to establish desired traits and then the crossing of two unrelated inbred strains. Generally the product of this cross shows amazing qualities, far surpassing those of the immediate parents and of the ancestors on each side. Geneticists call the effect "hybrid vigor."

These and related principles guided the work of Marita Mullan. From her original pair of wild mice she developed dozens of independent strains. She kept careful records of each individual and, following the example of Mendel, made tables showing the number of individuals in each generation with like characteristics. Her tables were far more complex than those listing the two factors of tallness and dwarfism in peas, however, because the several colors in mice are determined not by two genes but by a series. This vastly increases the number of possible combinations and adds to the interest and challenge of the game.

As Miss Mullan says, "The thrill of breeding unusual offspring is not the only appeal of genetics. Those who have crossword-puzzle minds will find that genetics on paper becomes a fascinating and challenging form of mental gymnastics. A simple knowledge of the kinds of genes and how they are distributed on chromosomes is all that one needs to commence dreaming up problems of inheritance and writing down the specifications for the new kind of individual you wish to withdraw from nature's reservoir. The chance combinations in this reservoir are not limited to color in mice. The study of the structural abnormalities of the skin and fur, for example, can be exciting-and sometimes amusing

"The most comical of all these mutants is the hairless. The hairless mouse spends the first two weeks of its life growing a full normal coat of fur and at this point cannot be distinguished from its normal brothers and sisters. Soon, however, the fur begins to drop out and the hair line swiftly recedes to complete baldness, so that in a few days the young mouse resembles a professor in a fur coat. Shortly the top of this coat is shed and the mouse seems to be wearing breeches. The final stage is perhaps the most amusing of all, for in a few days all trace of hair is gone except for a fringe about the haunches, and the mouse looks for all the world like a small, awkward ballerina. The entire loss is comparatively rapid and is completed in about 14 days. Thus four weeks after birth the creature has grown a coat and lost it-is finally as naked as a newborn baby. If there are no complicating factors, the mouse will soon regenerate a coarse fuzz which usually remains throughout its life. The first mice of this kind were caught in London in 1926.

"Some mice are not totally hairless, and yet are not completely furred. These have long fine fur which is much less dense than that of the normal mouse. In some the length of the fur is so reduced that it is necessary to use a magnifying glass to examine the quality of the fur.

"These strange characteristics are but a few of the interesting mutants which have appeared as the result of breeding two apparently uninteresting mice."

AS EARLY as 1814 Joseph von Fraunhofer, the father of astrophysics, placed a prism before the 1.2-inch lens of a theodolite and mapped the dark lines of the solar spectrum he saw, designating them with the now familiar letters. These are the Fraunhofer lines that give the stars the separate individualities of different human faces-individualities that are but dimly realized by those who observe only with a telescope. Unlike the telescope, the spectroscope reaches into a star and takes a sample of it. Paul W. Merrill of the Mount Wilson Observatories has said that studying a star by telescope is like "trying to guess the contents of a book from its size, weight and general appearance; while a spectroscopic observation is opening the book and reading it through line by line."

Today astrophysics, which deals with the physical and chemical characteristics of the stars, is the largest branch of astronomy; in fact, the astrophysical tail now wags the astronomical dog. Yet not one amateur astronomer in 100 attaches even a simple spectroscope to his telescope or seeks to become an amateur astrophysicist. True, much of astrophysics is abstruse, but not all of it. Getting started has been the chief obstacle.

A simple way to get a start in astrophysics is to build the little ocular spectroscope described by Roger Hayward's drawing on the next page. With it you can study the spectra of the brightest stars, including the sun, directly or as reflected by the moon. This spectroscope will show the more prominent lines of the solar spectrum when held in the hand and aimed at the sun. But when you insert it in the telescope in place of the eyepiece, take care not to look through it directly at the sun, for that can make you blind. Without the telescope the spectroscope may also be used on light sources such as neon tubes, a salted gas flame or a welder's iron arc.

It is called an ocular spectroscope because its diameter is uniform with the standard telescope ocular, or eyepiece. It is kept with the set of oculars and adds variety to their use. Its multicolored diffraction-grating spectra will also serve to satisfy the astronomically unsophisticated visitors whom all telescope owners occasionally have to entertain and who, seeing only with their eyes and not with their understanding, fail to be impressed. The ocular spectroscope will make your Aunt Emma say "Ah!" even though she may never have heard of Kirchhoff's three laws of spectrum analysis.

The midget spectroscope was designed and built by Ernst Keil, an amateur astronomer and professional instrument maker at the California Institute of Technology in Pasadena, Calif. As an avocation he has from time to time designed and built little ocular spectroscopes, including one for James Fassero, the author of Photographic Giants of Palomar, who uses it in his lecture demonstrations with the 100-inch telescope at Mount Wilson.

The achromatic lens of about two inches focal length may be obtained from war surplus for a dollar or two, or a plano-convex lens may be substituted with little optical loss. The only working dimension is the 1-1/4 inch outside diameter, a carefully machined sliding fit for the telescope drawtube. The other dimensions are those you choose. There are no "blueprints." Keil supplies only the little round gratings, which are replicas made by his own process, developed years ago and different from others. "The replica film," he writes, "is an integral part of the glass backing on which it is cast and is not a negative but a positive, giving the same distribution of light as the original." For a simple spectroscope a replica is as good as an original, and costs much less. The only way today to obtain a small original grating is to buy the costly laboratory spectrograph of which it is a part.

"The slit," Keil writes, "consists of two steel jaws made with care, their razor-sharp edges perfectly straight; see Amateur Telescope Making, page 248. The better the jaws, the sharper and more distinct will be the spectrum lines Their separation will depend upon the brightness of the star observed, but .01 inch should be suitable.

"The light from a star is gathered by your telescope and focused in the plane of the slit jaws. Entering the slit, it passes through the transmission grating, which disperses it into its colors, then through a lens that collimates the light (making it parallel) and magnifies the spectrum. In this spectroscope the grating is put behind the collimator, instead of in front of it, to protect the grating. Actual trial will prove that in this simple spectroscope it makes no difference on which side of the grating the collimator is placed, for the spectroscope is not intended for serious scientific research but only for demonstrating the elementary principles of spectroscopy.

"To put the instrument in operation, first rotate the grating-lens unit, which must have a sliding fit inside the outer tube, until the grating lines are parallel with the slit. Then slide it in or out until the slit is in sharp focus. Insert it in the telescope and move it in or out until brilliant spectra appear.

"One available replica has 7,500 lines per inch and makes a spectrum of great intensity but comparatively small dispersion. Another, with 15,000 lines per inch, has about twice the dispersion of the first but a less brilliant spectrum.

"The slot on the front of the spectroscope is at right angles to the slit and of such a depth that a filter placed in it will cover one half of the slit. Two spectra, one above the other, will then be seen simultaneously-one the original, the other an absorption spectrum. Gelatin filters may be had from the Eastman Kodak Company or you can use red or blue cellophane, obtainable at photography stores."

A less serious addition to the amateur telescope owner's set of eyepieces was made by Alan R. Kirkham. He built a Tolles solid eyepiece lens of crystal quartz which is doubly refracting and produces two images. Thus he could always reveal a "secret area" of the sky where all the stars were double. An "eyepiece" built by Leo J. Scanlon consisted of a spinthariscope mounted inside an eyepiece shell. This is a particle of radium compound in front of a fluorescent screen of zinc sulfide, set behind the magnifying eye lens. Thus he could always show "exploding universes" through his telescope.

IT IS believed that most of the secret methods of making replicas of diffraction gratings, about which amateur telescope makers often inquire as if this department knew the secrets, are minor variations on basic methods long ago made public in The Astrophysical Journal. In 1905 Robert James Wallace of the Yerkes Observatory wrote in that periodical that T. Thorp of England in 1900 was the first to describe a presentable replica. Over the original grating he flowed a thin film of oil and then a celluloid solution which he left to dry. He then peeled off the thin, tough film and mounted it face up on glass with gelatin and glycerin, lowering the film gently and gradually into place. In the same volume of the same periodical Wallace described his own method. He flowed especially prepared collodion over the grating, allowed it to dry, stripped off the resulting film and mounted it on gelatin-coated glass.

In The Astrophysical Journal for March, 1910, J. A. Anderson described his own method. In the collodion he dissolved certain unnamed gums, then placed the finished replica face down on glass and heated the glass. The solution oozed into the grooves and hardened as a negative cast of the replica and positive cast of the grating. He then dissolved the glass between the ridges with hydrofluoric acid gas. These published methods are believed to have been the basis of the secret methods.

In 1937 the Perkin-Elmer Corporation developed the concave replica grating shown in the illustration on the opposite page Its grating area is 2 by 2-1/2 inches. These gratings are superior to the ordinary low-priced replica. They are made as shown in the illustration at the top of page 90. At the bottom is the glass support for the original grating and on it is the aluminum film in which the actual grating was ruled. The grating is greased and given an evaporated aluminum film. A liquid plastic fill of Laminac is then added. The supporting mirror for the future replica is placed on top of this and the plastic is polymerized by heat. The replica unit is then parted from the original grating at the level of the grease. Very high-grade replicas have recently been developed by the Bausch & Lomb Optical Company for use in large spectrographs.

Interesting experiments have been conducted in England by Sir Thomas Merton and L. A. Sayce. A very fine screw-thread is ruled on a steel cylinder with a diamond. The cylinder is then coated with cellulose acetate. When dry, the resulting film is slit lengthwise of the cylinder, peeled off like the bark of a tree, flattened and used as a high-grade replica. Details of this development are reserved for future description.

The replica method has often been suggested as an easy solution of the large grating problem. Those who propose it apparently overlook the fact that a large replica calls for an equally large and non-existent original grating. There is, however, a way to make a large grating from a small one. It is the composite grating, an example of which is depicted in the illustration at the bottom of the next page. (Such a grating, composed of four 5-3/5 by 7-1/4 inch units, has been in use at Palomar Mountain.) This beautiful apple, however, is full of worms. While the spectrum from a composite grating is brighter in proportion to the number of units compounded, the grating has no greater resolving power than a single unit unless the demands of optics are met by a degree of mechanical precision that has not yet been attainable. Just how would the proponent of this method accomplish the following? He must manipulate the flimsy films on a backing of glass in such a way that the grooves in the upper right-hand unit in the illustration are in phase, or in step, within one millionth of an inch with the grooves in the unit at its left, regardless of the separation between the replicas. If he can accomplish this, his next job is to position the lower units similarly and make them colinear with all the upper units. Unless this geometric perfection of the whole is attained, the grating will remain essentially only a one-unit bright grating. The second complication is the exceedingly stiff optical requirement. For a 12-inch unit the glass backing must be flat to within a millionth of an inch, and for a 24-inch unit, to within one two-millionth of an inch. The composite is not a practical method of making a large diffraction grating.

THE refracting telescope has so many fine qualities that amateurs would be just as keen to make it as the reflecting type, if it did not require additional equipment: a lathe, spherometer, optical flat, preferably iron tools. It also calls for optical glass. High-grade, dependable optical glass has always been difficult for the individual small purchaser to obtain. Most of the glass types in the manufacturers' catalogues are sold only at wholesale or are monopolized by the manufacturers themselves. The amateur could have fun if he could range all through these attractive and expensively produced lists and order a little of this glass and a little of that and experiment with uncommon types. Instead, only two or three types are dealt out, sparingly and in but a few sizes. This throws the amateur's nose out of joint. On the other side of the ledger, the cost of paper work and correspondence (especially with amateurs!) renders these small retail sales uneconomical. The brief lists of glass are a compromise with the economics, made because the manufacturers personally admire the amateur's enthusiasm.

Under an arrangement made by this department, two additional types of glass in five diameters are now added to the amateur's limited list. They are manufactured by Chance Brothers, Ltd., of Birmingham, England. Chance Brothers' optical glass was used by W. F. A. Ellison and is the basis of his exposition of objective lens design in Amateur Telescope Making. The U. S. agency for Chance glass is no longer in existence, but no import license or other red tape complicates the purchase of a small pair of telescope blanks from abroad. The price is remitted by foreign postal money order. When the parcel arrives, the local letter carrier collects the import duty and leaves the parcel. The British prices have been adjusted to the 50 per cent ad valorem duty.

The following are the prices of crown and flint pairs: 3.3-inch postpaid at $6.33, 3.8-inch at $9.33, 4.3-inch at $14, 5.4-inch at $23.33 and 6.4-inch at $38.66, exclusive of import duty. The stated diameters are nominal, the respective clear apertures being 3, 3-1/2, 4, 5 and 6 inches. For each size there is a choice of crown blanks: a hard crown with refractive index 1.519, dispersion 60.4 and a borosilicate crown with index 1.518, dispersion 64.1. The same dense flint, with indices 1.620 and 36.1, accompanies either crown. The glasses are "first quality", meaning, guaranteed completely free from striae. They are accompanied by full optical measured readings and melt number. The surfaces are smooth-ground.

Colonel Alan E. Gee, a widely known amateur and professional optician, points out that the hard crown and dense flint combination gives a secondary spectrum f/3200, the other combination, f/2500. Since the average criterion used in professional work is about f/2400, he considered the first combination "mighty good."

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.