TELESCOPE MAKERS who plan lightweight and spindly-legged tripods are urged to study the proportions of the tripod in Figure 1. With the heavy mounting and 4" refractor the total weight is 288 pounds. John McLennan, 1424 Oakwood Ave., Akron, Ohio, is the maker and he says the four main castings are of bronze with proportion 85 percent copper, 15 percent tin. The rugged axes are 2" and 2-1/8" in diameter, respectively, and both turn in precision ball bearings filled with grease.

There are setting circles on both axes and the polar axis has a slip ring for setting on sidereal time when starting to observe.

"When making the working drawings for the mounting I did not think it would work out so heavy," McLennan writes, "but I am very well pleased with it. I am not a machinist-just a home-trained mechanic."

Not only is this mounting rugged but it also is clean, regarded as a piece of design. Excellent proportion.

REFRACTOR making is beginning to catch on. A decade ago it was being discouraged for amateurs. When "ATM" was first put together, largely from Ellison's book "The Amateur's Telescope," the full American reprint rights of which were purchased, Ellison's chapters on the refractor were omitted. The reflector, not the refractor, is the amateur's meat, we said then. By 1928 (second edition of "ATM") we had softened enough to insert a guarded note on "The Objective Lens" (page 258, 2nd ed.), pointing out the difficulty of the work, and that same note, only lukewarm on refractors, was reprinted in the third and fourth editions (page 333). Last year, when we reprinted the fourth edition, we slipped out that old discouraging note and substituted one that now encourages the amateur to make a refractor. Thus, 13 years produced a complete about face. Just a diplomatic way of saying that in 13 years the amateur has steadily risen, risen, risen in his ability.

Speaking of refractors, the next item may throw some added light on them-literally.

IN Figure 2 are two pairs of superposed, flat-faced disks of flint glass. You can see far more clearly through the pair on the right. This is because the latter have been coated with the new metallic fluorides, and the day may come when amateur objective lens makers will send their components away to be treated, or do it themselves if equipped. This should add about 20 percent to the light transmission through a two element objective. Refractors give steadier images than reflectors but lower illumination, hence this will be a big help. Here is the background, assembled from different sources, published and unpublished.

In 1892, H. Dennis Taylor, a noted British lens designer, began to take notice of the effect of tarnished films on lenses when some flint elements were returned for repolishing. His tests showed that, if a photographic plate was exposed under identical conditions, through a tarnished and an untarnished lens, more light would reach the plate through the former. Said he: "Whereas a thin plate of dense flint glass of the type usually used for objectives will, when freshly polished, reflect back from its two surfaces about 11 percent of the light falling upon it, and transmit 89 percent, the same plate when tarnished . . . will reflect back only about 5 percent, and transmit 95 percent." He therefore attempted to tarnish lenses artificially by immersing them in various solutions and found that hydrogen sulfide and alkaline sulfides reduced reflection appreciably.

In 1916, Kollmorgen, "acting upon the hints contained in Taylor's description, experimented a good deal with different chemicals along the same lines and found means of oxidizing most of the glasses used in optical work," according to a paper delivered before the Illuminating Engineering Society.

At about the same time, Dr. Hermann Kellner, who was head of the Scientific Bureau of Bausch and Lomb, was working along similar lines. He mentioned his experiments to Dr. F. E. Wright, during the latter's labors at the Bausch and Lomb plant in 1917, as a member of the group of silicate chemists aiding the Government in securing its requirements. Dr. Wright began a series of experiments with Dr. J. B. Ferguson. Although these were never concluded, reflectivity was definitely decreased and Dr. Wright offered three hypotheses to account for the phenomenon the most likely one being that, in the process of etching by the attacking solution, the surface becomes covered with minute pits which are small compared with the wavelength of light.

In 1936, John Strong described a physical method of coating films to produce the same effect as the chemical method. His process involved the evaporation of metallic fluorides in a high vacuum. He described the use of a film of decreasing index of refraction which starts out with the full index next to the glass. This was done by depositing a film of fluorite, solid below and porous above. Agreeing with the Wright hypothesis, he states that "the grain of this porosity is small compared with a wavelength of light, as evidenced by the fact that it does not scatter light." With such a film he noted decreases in reflection of from 54 percent to 85 percent, depending on the angle of incident light.

Since Strong's work several physicists have used this technique, among them being Cartwright and Turner who have used various metallic fluorides to produce imperceptible films on glass to increase the transmission of light.

Simultaneously, Dr. Katherine Blodgett, working on a project originally instituted by Dr. Irving Langmuir, employed films of barium stearate and other fatty acids to form a thin molecular surface on glass. The glass was dipped into a tank of liquid on the surface of which an insoluble soap one molecule thick was placed. As the glass was dipped down, one layer of film was attached, and on withdrawal another layer was applied, each full immersion adding two layers, each one molecule thick. It took 44 successive layers to build up a film to a thickness of a quarter wavelength of light. Dr. Blodgett came to the conclusion that, except for the loss of light by absorption in the glass itself, film treated lenses could be made to transmit 100 percent of the incident light.

The metallic fluorides, however, are much tougher films than the fatty acid types. In the long series of investigations conducted by Dr. W. B. Rayton in the Bausch and Lomb laboratories during this period, it was evident that a combination of two processes could be used to greatest advantage. On lenses having an exposed outer surface a corrosive chemical process is used in which oxides of high refractive index are removed from the surface, leaving an invisible structure of silica, while the inner glass-air surfaces are coated by the deposition of a metallic fluoride in a high vacuum.

In both processes the coating is held to a thickness of a quarter wavelength of light, or about four millionths of an inch. The film must be intermediate in refractive index between air and glass. Since both the film and the lens surface reflect light, it is necessary that the crests of the waves from one beam shall fall into the troughs of the other. Thus, being out of phase, the waves neutralize each other and reflection is decreased or entirely eliminated. The missing radiation reappears in the transmitted beam which has been shown to contain as much as 99.6 percent of the original radiation.

From 4 to 6 percent of the incident light is lost at each glass-air surface of a lens, the precise amount depending upon the type of glass used and the color of the light A crown glass-air surface reflects about 4 percent of the light, whereas flint reflects 6 percent. It will readily be seen that, in a lens with a number of elements, or in optical instruments with complex systems of optical parts, light transmission can be increased tremendously. Equally important is the reduction of halo and the improvement in image contrast.

The new coating processes are at present being confined to a limited number of products but the progress so far attained offers a wide field for experiment which may prove a great step forward in optics. One advanced amateur who has seen some experimentally coated binoculars says the difference is striking, as binoculars have from 10 to 15 surfaces, giving fine opportunity for the beneficial effects to pile up.

HIGH cost of gas for melting quart was one big reason for giving up the original intention of making the 200" mirror disk of that material, as the melting point of quartz is very high. Behind the following communication from John Ferguson, consulting engineer, 4 E. 194th St. Cleveland, Ohio, may be something new and significant.

"I send you," he writes, "a photograph (Figure 3) of a paraboloidal mirror which is unique. It is made of borosilicate glass is 15" in diameter, 3" deep, and of cellular construction. I believe it to be the first mirror ever to be made from glass which was melted electrically, as distinguished from fuel fired furnace melting. The process of melting the glass was developed by the writer, the heat for melting being generated by the flow of electric current through the glass material itself, acting as a resistor.

"Glass at normal temperature is a good insulator, but it is capable of conducting current when heated. In this respect it is classed as an unstable resistor; its resistance becomes progressively lower as it is heated. Being a liquid, it cannot burn out, as solid resistors will do at high temperatures, so the melting temperatures can be pressed up to as high values as the refractory containers will withstand, and one of the advantages of this is that very refractory glasses can be melted.

"It may, of course, occur to the reader to wonder how the glass ingredients-sand, borax, soda, lime and so on-are brought to an initial temperature condition where they will begin to conduct the current in appreciable quantity. They become conductive at 500 degrees to 600 degrees C. and there are several methods of producing this temperature; after which the flow of molten glass and the entire process becomes continuous.

"The electrical process of melting glass appears to promise glasses of superior characteristics for mirrors. Glasses have been successfully melted with a silica content as high as 87 percent, with quite low expansion factors. The nature of the operation also will lend itself to the production of quite large mirrors at considerably less cost than has heretofore been possible."

It is said that the process is already in use by two large American glass companies making glass for purposes other than mirror disks. A detailed account is to appear in the Journal of the American Ceramic Society. What effect this process may have on the availability of mirror disks having a higher silica content, and therefore less expansion effect, than those now available, this department knows not at the present writing. Pyrex disks of the sizes most commonly sold to amateur telescope makers (under 12-l/2") contain 80 percent silica, the larger ones 87 percent.

TO all quarters of the earth goes Scientific American and many of its readers obtain "ATM" afterward. One is the Rev. Emil W. Menzel, of the American Evangelical Mission, Bisrampur, C.P., Via Bhatapara (B.N.Ry.), India, whose telescope is shown in Figure 4. It has a 10" mirror made of a port-hole glass and the mounting is of the open, "spinal column" type, the neat column being a piece of steel channel. It is equipped with setting circles. "Since I live in the jungle it has been hard to get machined parts," its maker states, "and so the mounting is about as primitive as it can be and still do service. The long focus gives plenty of exercise climbing on boxes and tables. The telescope is mighty good company on lonely, hot nights when you sleep outdoors under the stars with it at your bed-side." Note the low angle of the polar axis, designed for the latitude of India's Central Provinces, in about 20 degrees North.

ALL mirror makers have been asked whether the rouge they use is "the same kind." One answer might be obtainable simply by inviting the ladies in you circle to apply some of your rouge to their faces; no doubt they just wouldn't. Or by trying some of theirs on your mirror. Even so, how many users of red optical rouge, which is ferric oxide (Fe0), can say with assurance what, if anything, the two genders of rouge have in common, besides the color red rouge being simply the French word for red (which, incidentally, makes one ask why some people call Levigated Alumina "white rouge").

These weighty questions having kept your scribe awake nights for the past dozen years, he asked Horace H. Selby, author of the chapter on making optical rouge, as well as other chapters in "ATMA," and who is a professional chemist, for the answers. Here is what Selby replied: "You got me curious with your inquiry: so I got some samples-three kinds-and went to work. Was I surprised! Not a milligram of red in the lot! In two of the seven shades, I found approximately 3 percent of a yellow iron oxide-presumably ocher. In general, all were dried cakes of gums (acacia, tragacanth or India) holding together mixtures of magnesium silicate ("French chalk," "talc" and so on), aluminum silicate (colloidal kaolin or clay), calcium carbonate (chalk), zinc oxide and colors. Of the colors, I identified the following: ammonium carminate, carminic acid, a fluorescein salt, yellow ocher, a nitrofluorescein-possibly eosin scarlet. Some other colors were present which I couldn't spot easily. They were lakes, and some were blue, some yellowish. The preponderant colors were of the cochineal group, however (carminic acid and salts and crude carmin.) "

Accompanying this learned report, Selby sent a table showing that seven rouge compacts were tested by him for 20 components each. To keep the peace we suppress the makes; you wouldn't know them if you saw them, anyway (or would you?). Selby says he did about $100 worth of work on the job. Knowing, however, that he would not be interested in mere money, we have decided to award him, instead, and on be" half of all amateur telescopedom, an honorary degree. Now the degree of J. U. D. (Juris utriusque Doctor) stands for "Doctor of Both Laws," that is, canon and civil, so why not create for Selby the degree of R. U. D. (Rubrorum utriusque Doctor), which means "Doctor of Both Rouges". So be it, and Selby is ennobled for his rouge research.

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