NANCY RENTSCHLER, WHO GOES TO high school in Mayfield, Ohio, writes that she cannot claim any hobby because she enjoys so many. She plays the piano and the drums, knits, cooks, enjoys dry-fly fishing, does volunteer hospital work and roots fervently for the Cleveland Indians. She is also a consistent winner of blue ribbons at science fairs. Collecting this year's blue ribbon involved, among other things, learning how to pick up a mouse without getting nipped.

Miss Rentschler had to master this trick in order to study the metabolism of mice. Last January, while looking for a project to enter in her school science fair she came across a textbook diagram of an apparatus to measure animal metabolism. This aroused her curiosity. She writes: "I didn't know very much about metabolism, but it seemed to me I could learn if the apparatus could be scaled down to the size required for a mouse. The apparatus would also make an exhibit worthy of the 220 entries in our science fair. I began to work on the apparatus late in January, and performed my first experiments with it about a month later.

The mice I used were purchased through a pet shop. They had been inbred for three generations. At first I found it a bit difficult to handle them, but soon learned to pick them up by the tail. After a week or so the mice became quite tame, although occasionally one would lose its temper during an experiment and try to bite the experimenter.

"For the purpose of my experiments I divided 15 mice into four groups, three in one group and four in each of the others. By placing each group on a diet or medication which differed from that of the others, I could study the effects of these differences on the metabolism of the animals. I followed the experimental method devised by the noted British physiologist J. S. Haldane in 1890. The apparatus consists mainly of an animal chamber and five flasks of chemicals interconnected by tubing so that a controlled stream of air can flow through the system [drawing left]. The purpose of the apparatus is to measure the amount of oxygen taken up by the animal, and the amount of carbon dioxide expelled. The ratio of oxygen inhaled to carbon dioxide exhaled by the animal during a given period indicates the rate of its metabolism, and is called the 'respiratory quotient.' This quotient varies with the diet of the animal. When the animal is fed a carbohydrate such as sugar, the ratio is 1. When it is fed fats, the ratio varies slightly with the composition of the fat but averages .7. The ratio for proteins also varies, but averages .8. The ratio of alcohol is .667. The respiratory quotients of normal animals under average conditions usually lie between .72 and .97.

"Each flask of the apparatus is fitted with a rubber stopper and two glass tubes about half an inch in diameter. One tube reaches to within an inch of the bottom of the flask and the other just passes through the stopper. Air entering the flasks through the longer tubes is exhausted through the shorter ones. The first and fourth flasks in the series (not counting the animal chamber) are filled to a depth of about three inches with soda lime, which absorbs carbon dioxide. The second and third flasks contain the same amount of calcium chloride. The fifth flask is charged with pumice stone and sulfuric acid. These last three flasks absorb water vapor. Ideally all three should contain pumice and sulfuric acid. I found the pumice difficult to prepare, so I made enough for one flask (to satisfy myself that I could prepare it) and 'made do' with calcium chloride in the other two. The pumice is used in lumps about half an inch in diameter. Mine came from cosmetic counters, which proved to be a costly source. I learned later that chemical supply dealers list pumice at 50 cents a pound. The stone is activated by heating it to redness with an acetylene torch and dropping it, while it is still hot, into concentrated sulfuric acid. The excess acid is then allowed to drain off. The soda lime is prepared by mixing lime with a solution of sodium hydroxide in the proportion of one ounce of sodium hydroxide (by weight) to two and a half ounces of water (by volume). Lime is added until the mixture becomes dry. The powder is then separated from the coarse particles by means of a fine sieve and discarded. Large lumps are broken down. It is the intermediate fragments —those which pass through a sieve of five meshes per inch—that are used for charging the flasks. The absorbing power of soda lime does not last long, and I had to make additional batches as the experiments progressed.

"My animal chamber was a two-quart canning jar. I found it necessary to shield the exhaust tube of the chamber to keep it from pinning the mice. Before I added the shield, this happened several times, spoiling the experiment and injuring the mouse. The shield is merely a short length of rubber tubing with a slit or a few holes cut in it. It is slipped over the shorter glass tube inside the chamber. No damage is done when a mouse brushes against the end of the tube because the slit provides a second exhaust port.

"The entire system must be airtight. Close-fitting stoppers should be used and all joints coated with either wax or plastic cement. The rubber tubing should be as short and straight as possible, and should be tightly fitted to the glass tubes. Air was pulled through the apparatus means of an aspirator attached to water faucet.

"Air normally contains about 3 per cent carbon dioxide and a varying amount of water vapor. Both are removed by the first and second Flasks. Thus air free of water vapor and carbon dioxide flows into the animal chamber. The animal inhales oxygen and exhales carbon dioxide and water vapor. The latter are absorbed by the remaining flasks. The increase in weight of the third flask indicates the amount of water vapor given off by the animal. The fourth and fifth flasks measure the amount of carbon dioxide (which reacts with the soda lime in the fourth flask to form carbonic acid). The fourth and fifth flasks must be weighed together because the soda lime may give up moisture to the dry air and thus lose weight,

"In setting up the apparatus for a test run, the last three flasks are weighed, the fourth and fifth together. The animal is then placed in the chamber, which is stoppered and weighed. The test run is timed from this moment. The chamber is now connected to the apparatus and the air pump started. I ran the mice in each group for a total time of one hour. At the end of this period the pump is stopped and the chamber removed from the apparatus, stoppered and weighed again. The third, fourth and fifth flasks are also weighed.

"The respiratory quotient may now be calculated. The combined weight of the 'mouse and chamber at the beginning of the run minus their weight at the end of the run equals how much weight the mouse has lost. The weight of the third flask at the end of the run minus its weight at the beginning equals the amount of water absorbed by the calcium chloride and lost by the mouse The weight of the fourth and fifth flasks at the end of the run minus their weight at the beginning equals the amount of carbonic acid formed. The total weight of water and carbon dioxide absorbed minus the loss in weight of the mouse equals the weight of oxygen absorbed. The respiratory quotient is determined by multiplying the weight of the carbonic acid by the fraction 32/44 and dividing the result by the weight of oxygen absorbed. The quantity 32/44 is ratio of the molecular weight of oxygen to that of carbon dioxide. Its use in the equation indicates the amount of carbon dioxide represented by the carbonic acid.

"I used two of my four groups of mice to study the effects of diet on metabolism. With the other two groups I instigated the metabolic effect of the activity of the thyroid gland. The first group of four mice was given only water. Although mice normally live about nine days without food, these died after four days. It is likely that they contracted pneumonia because their resistance was low. Their respiratory quotient dropped slightly from the beginning of the, experiment but stayed within the normal limit of .7 to 1 for the first three days it plunged sharply just before the animals died. Oxygen consumption, however, decreased at a constant rate throughout the period of observation. At the conclusion of the experiment I plotted graphs of both oxygen consumption and respiratory quotient [Figure 2 ].

"In the second group of mice a 17 per cent solution of ethyl alcohol was substituted for water. Each-mouse also received one gram-of rabbit pellets per day, beginning on February 26. On March 4 I found the mice shivering and huddled together in their cage. Fearing that they might die if a test were attempted, I fed them immediately and wrapped them in warm rags. Their ration was doubled for two days and then lowered to a gram and a half on the third day. One mouse died on March 9 and another the following day. I attempted to study the remaining two in the metabolism cage but their rate of respiration was so low that no results were detectable at the end of a two hour run. According to a doctor friend whom I consulted during the experiments, the mice in this group died of semi-starvation and extreme intoxication ending in pneumonia and shock. The oxygen consumption of the group increased sharply during the first three days of the test, dropped for two days and then climbed gradually to a peak just before the animals died. The respiratory quotient, although low, remained within normal limits almost to the end. A restricted diet with an excess of alcohol causes fat to accumulate in the liver and retard some of its functions The results of this experiment were also plotted in graphs [Figure 3].

"On February 27 a group of four mice was started on a mixture of powdered rabbit pellets into which .1 per cent of desiccated thyroid gland had been mixed. Because this medication stimulates the thyroid the mice, which were permitted to eat as much as they would consume, gained weight steadily during the experiment. At one point the apparatus developed a defect and two mice suffocated. I continued with the remaining pair. Oxygen consumption appeared to drop during the final days of the experiment, but this too may have been due to a defect in the apparatus. The respiratory quotient remained below normal almost from the beginning and indicated no trend [Figure 4].

"The final group of three mice was injected with 100 microcuries of radioactive iodine (I-131) on February 26. This proved to be an overdose which destroyed the thyroid gland in about four weeks. The injections were administered in a medical laboratory, where the mice were kept for three days. Upon their return they were supplied with as much water and rabbit pellets as they would consume. The outward appearance of the group did not change during the period of the test. Oxygen consumption fell gradually during the first eight days and then increased to about double the minimum value on the 14th day. Thereafter it dropped more or less gradually to 10 per cent of its initial value on the 22nd day. The respiratory quotient also varied widely during the experiment but showed a gradual decrease until the final day of the run, when it shot up from a near zero value to normal [Figure 5 ]. These changes were expected because the thyroid was slowly deteriorating and its production dropped in proportion.

"These experiments have been the most rewarding, although the most difficult, of my science projects to date. Last year I worked with blood sugar, making my own tests. The year before I attempted a series of genetic experiments with mormoniella wasps. Although I cannot claim biology as a full-time hobby, it has interested me for as long as I can remember. Few hobbies, it seems to me, confront the amateur with such a variety of challenges."

If you like to while away the time performing experiments that do not require apparatus, you will find much solace in the mysterious operation of human vision. You already own a pair of excellent optical instruments for such experimentation: your eyes. With these, plus pencil and paper, you are all set. If, in addition, you happen to have a pair of pocket mirrors, a couple of short-focus lenses and a stereoscope, you can really astonish yourself.

As a starter, draw a rectangle about three inches wide and two inches high and divide it by a horizontal line a quarter of the way down from the top. Then draw a series of disks diagonally across the bottom portion which gradually diminish in size, as shown in the illustration at the right. Common sense tells you that you have made a flat. drawing. Yet your brain insists that it is a three-dimensional representation–especially if you judiciously shade the disks. You get the impression of a series of spheres which run from the foreground to a "vanishing point" on the horizon. It took the painters of the Renaissance a century to perfect this trick of representing three-dimensional reality in two dimensions. The "projective geometry" on which it is based is today an integral part of physics.

There is another method of representing three-dimensional reality in two dimensions which creates an even more dramatic visual impression. Make two rectangles, each an inch and a half wide by an inch and a quarter high. They should be spaced about two and a half inches apart from center to center. Now draw a horizontal line through the center of each, dividing the rectangles into two equal parts from top to bottom. Next make a shaded, quarter-inch disk, precisely in the center of each horizontal line. Flank the disk in the drawing at left with an identical pair of disks spaced 3/16 of an inch from it. Make a similar pair on the horizontal line of the drawing at right, but space them 5/16 of an inch from the middle disk. To the casual observer there is certainly nothing in the pair of drawings to suggest relief in three dimensions. But when you view the drawing in a stereoscope, which causes the pair of rectangles to blend into a single image, the disk at left is seen as a sphere floating in space above the plane of the paper. The center disk will appear as a sphere in the plane of the paper while the right one will seem to float in space behind the paper. With a little practice you can observe this effect without a stereoscope. Locate the drawing about two feet away and place the index fingers of both hands just outside your eyes. Now, while continuing to look at the drawing, move your hands slowly toward the drawing. Your left eye will see the tip of your left index finger and your right eye the tip of your right finger. As your hands advance, you will become conscious of four rectangles on the paper. Your brain is accepting the independent images presented by each eye. Now the inner pair of images will gradually overlap. Finally they will blend. When this is accomplished, transfer your full attention to the fused image. It will appear in three dimensions, just as though it were seen through the stereoscope. This is called "wide-eyed" stereoscopic seeing. You can also see the drawings in three dimensions without a stereoscope by the "cross-eyed" method. To achieve this you use only one finger. Place the drawing about two feet away, as before. Now put the tip of one index finger on the bridge of your nose and while looking toward the drawing slowly move your finger toward it, focusing your eyes on the tip. Again you will become conscious of four rectangles on the paper. Gradually, as your finger advances, the innermost pair will fuse as in the wide-eyed method and you will see the center drawing in three dimensions. It will differ from the wide-eyed view, however, in two major respects. The fused image will be smaller by about a third because, among other things, the optical path between the eyes and paper is now longer. You will also observe that the two outermost spheres will have exchanged position. The sphere at right now floats in front of the plane of the paper and the one at left behind it! The relief has been inverted.

Roger Hayward, who illustrates this department, has amused himself at odd moments for the past 30 years by learning to make stereoscopic drawings and investigating stereovision. "Three-dimensional drawings are not nearly as hard to do as one would imagine," he writes. "This is due to the tolerance of the eye, provided, of course, you understand what you think you see. If what you appear to see is inconsistent in geometry, that is, in perspective, or inconsistent in the effect of light and shade, the brain is apt to reject the stereoscopic effect. The picture will lack the apparent relief of 3-D and will appear relatively flat. This is illustrated by the pairs of drawings at the right. The top pair, which is drawn for cross-eyed viewing, shows a prism lying in front of a screen. Both objects are in the foreground of a long room. The scene is lighted from the upper left; part of a window can be seen in the rear wall. The bottom stereo pair shows the same scene but is drawn to be viewed either by the wide-eyed method or through a conventional stereoscope. Let us assume that you either have access to a stereoscope or have mastered the art of wide-eyed viewing. On examining the drawing you will see the folded screen with a black band along the top and near the bottom. Part of the screen has been folded back and can be seen over the top of its front portion. The prism extends out in front and the screen casts a shadow on the right wall of the room. All this has the effect of a scene viewed in normal perspective with a self-consistent pattern of light and shade, although the pattern is a pretty conventional one.

"Now change to either cross-eyed viewing or substitute the upper drawing .for the lower one in the stereoscope. The relief will appear in reverse; near objects seeming smaller than far ones. The folded portion of the screen will seem nearer than the front part, which does not make sense. Other parts of the scene seem to be linked in a fuzzy sort of jumble. From thus experiment one can conclude that binocular vision is destroyed when violence is done to the principles of perspective. A near object which partly obscures a far one is nonsense which the mind simply refuses to accept. But if a figure is consistent in perspective and in light and shade, especially if the figure is unfamiliar, the mind will accept it as representing reality, however exotic the forms may appear.

"It is possible to investigate some of the limits within which the mind will accept misinformation from the eyes by means of an instrument called the pseudoscope, a binocular-like device which enjoyed brief popularity shortly after Sir Charles Wheatstone invented the stereoscope early in the 19th century. Diagrams of four versions of this instrument appear in Figure 9. Pseudoscopes alter the way in which the eyes normally present information to the brain. Some versions interchange the eye positions, in effect causing the left eye to see from the position of the right eye and the right eye from the position of the left. Others combine this interchange with image inversion, exaggerate the spacing between the eyes and so on.

"A pseudoscope can easily be made by holding up two hand mirrors, one somewhat to the left of the left eye and the other in front of the right eye. The angles at which the mirrors are held should be such that the image reflected from the mirror at left is directed into the right eye by the mirror at right. In effect, this causes the right eye to see from a position somewhat to the left of the left eye. The defect of this arrangement is that the optical path for the right eye is longer than that for the left; hence the image presented to the right eye is abnormally small. For objects at a distance of 10 feet or more the difference in image size is of little consequence, however, because of the curious fact that good binocular vision can be had even if one of the images is quite distorted. Accordingly an interesting inversion of relief will appear when the mirrors are adjusted so that the doubly reflected image fuses with the one seen normally by the left eye. The optical arrangement of a pseudoscope of this type is shown at the top of the illustration in Figure 9.

"Other versions of the pseudoscope can be made with prisms, with two sets of mirror pairs or by mounting a pair of short-focus lenses in the cardholder of a conventional stereoscope. Optical arrangements for these are shown as the second, third and fourth drawings in the illustration above. Holders for the optical parts need not be elaborate; I merely laid the prisms of the second arrangement between two pieces of wood, for example, and held the whole thing together with rubber bands. The mirrors of the third device may be stuck to a small board with sealing wax and adjusted for image fusion while the wax is still soft.

"Martin Gardner, who edits the Scientific American department 'Mathematical Games,' first called my attention to the stereoscope-pseudoscope represented in the fourth drawing, and suggested an interesting experiment for it– viewing the effect of a small ball rolling inside a round-bottomed bowl. With no shadows to indicate perspective, the mind accepts the inversion perfectly. Released at the edge of the bowl, the ball promptly climbs up one side of what appears. to be a mound, rolls down the other side and promptly returns. Finally, after a number of diminishing oscillations, the ball comes to rest on the summit of the mound!"

Bibliography

APPLIED PHYSIOLOGY. Samson Wright, Humphrey Milford and Oxford University Press, 1929.

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