Popular Science, marraskuu 1933
By Morton C. Walling
When you leave the world of ordinary things behind and, with the aid of your microscope, go on an exciting journey into the Land of the invisiblem you find your trip enlivened by color. The green chlorophyll of plant tissues, the red corpuscles of blood, and the colored crystals are a few examples of natural coloring.
The most important thing about color in microscopy, however, is the fact that you can use it to help you see things that otherwise would be indistinct or invisible. if you make photomicrographs, you cannot escape for long the use of color.
The process of employing color to improve the performance of your lenses is so simple that you will encounter no difficulty whatever in applying it. You simply insert a piece of colored glass or other colored, transparent material into a beam of artificial light focused to illuminate the object you are examining. Sometimes you can use two or three colors at once. The colored glass is called a screen or filter.
There are several reasons why a microscopist uses filters. For one thing, the lenses of many microscopes perform better in light of one color. Another reason is that, by using light of certain colors for illuminating specimens that have been treated with a dye, maximum contrast or detail can be brought out as desired. A third benefit lies in the fact that blue or green filters make long observations less tiring on the eyes. A fourth reason is that, in making photomicrographs, filters can be manipulated to control results. For instance, a section of yellow whalebone photographed by blue light would be rendered with great contrast, while red light would bring out the detail of the structure.
If microscope makers had but one color of light to contend with, their lives would be far easier. As it is, they must create lenses that are to be used in white light; and white light, as you know, is a mixture of many colors each of which behaves differently when passing through a lens. Laboratory microscopes, having achromatic lenses, are corrected for two colors (with reference to chromatic aberration).
This means that the lenses bring two colors of the spectrum to sharp focus, while other colors are not so sharply focused. Higher-grade instruments, having ap och romatic objectives, are corrected for three colors. As for the cheaper microscopes, it is doubtful if the lenses will bring more than one color of light to sharp focus at one point. The presence of color fringes about images indicates that perfect color correction is lacking.
The writer has a microscope that cost, when new, about $10.It magnifies to 125 times. Simple tests with a set of three-color photographic filters, that is, pieces of gelatine stained scarlet, green, and blue-violet and known to photographers as the "A," "B," and "C" filters, revealed that the microscope focuses all three of these colors at different points.When the image was rendered as sharply as possible by daylight, the insertion of any one of the filters into the beam of light falling on the mirror caused an apparent shift of focus. Refocusing would make the image sharp when the filter was in use. Then, if the filter were changed, refocusing was necessary. You will find that some filters, particularly when two of them are used together, absorb so much light that you may have to increase the illumination. On the other hand, filters reduce to a comfortable level the brilliancy of illumination that otherwise would be undesirable.
There are so many factors involved in the use of filters that no set of rules can be given in the space available. Colro of the object being examined, correction of the microscope lenses, amount of detail or contrast desired, all must be taken into account. it is not always desirable to use a filter. A good microscope will perform well in white light. However, many microscopists find that pure white light is tiring to the eyes, particularly when they spend hours on end watching the antics of an active paramecium or rotifier. By inserting a blue or green filter into the light beam, such fatigue can be avoided or at least reduced.
Many microscopes come equipped with a blue viewing filter that slips into a holder below the sub-stage condenser. You can purchase blue or green filters in gelatine form or as gelatine cemented between glass. A flask of copper-sulphate solution, placed in the light beam, is preferred by some workers as a means of making the light easy on the eyes.
You need not go to a lot of expense to procure a set of visual filters. Although expert microscopists probably would shudder at the idea, you can build up a useful collection by procuring pieces of colored cellophane or similar material, bits of colored glass, and old photographic filters. Many store owners can give you pieces of the transparent material they use for coloring the light from their show-window illuminators. You can stain bleached out photographic plates or films with household dyes. Use either a plate that has not been developed but has been cleared in hpyo fixing bath, or one that has been bleached by any of several methods, such as immersion in tincture of iodine followed by clearing in ordinary hypo solution.
At a cost of about ten cents per square inch, you can purchase special microscope filters in gelatine form. These are made by dyeing thin sheet gelatine. They must be handled with care because the gelatine will be damaged if touched with the fingers. Two- or three-inch squares of gelatine are preferable. There are a number of visual filters obtainable in this form.
In addition to the gelatine filters, you can buy numerous photographic filters. There is available a set of nine two by two-inch gelatine filters for photomicrographic work, at a cost of about $3.10. Most of these can be used as visual filters.
Filters made of gelatine, cellophane, and other fragile material should be mounted between glass or in cardboard frames to prevent damage when handled. A good way is to cut pieces of glass to a size about one-eight-inch larger all around than the filter itself, and then sandwich the gelatine between two glass pieces whose edges are bound with lantern-slide tape. This method is satisfactory for all kinds of microscope filters because they are placed in the beam of artificial light and therefore do not interfere with the light rays after they have left the object or passed through the lenses.
For holding the filters, you can construct a little stand that is adjustable to almost any position. Make a wood holder by cutting grooves in two blocks measuring about one and one-half inches long, one inch wide and five-eights-inch thick. Cut the grooves about three-sixteenths-inch deep, and space them one-half-inch apart, measured on centers. Fasten the blocks to a base piece five-sixteenths-inch thick, one and one-half inches wide and long enough to permit the filters to be slipped into the grooves or removed without binding. Walnut is one of the best woods to use for the holder.
Procure two three-sixteenths inch brass rods about five inches long, a piece of heavy iron such as a two-inch pipe cap, a piece of three-fourths inch brass rod one inch long or a brass bar or similar dimensions, and two stove bolts with washers soldered into the slots of their heads to serve as handles or grips for turning.
Mount one of the rods in the center of the pipe cap, and insert the other into a hole bored into the end of the filter holder. Drill two holes through the short length of brass rod, at right angls to each other; drill two more holes into the ends of the rod, so that they intersect the first two. Tap the end holes for the stove bolts. Slip one rod into each of the untapped holes, and use the bolts for locking the brass block in position. You thus have an arrangement that is simple and at the same time adjustable as to height, distance of the filters from the base, and angles of the filters. Lacquer the wood parts and the iron base.
Just one more item and your filter equipment will be complete. You will, of course, need a source of artificial light htat can be focused and projected through the filters to the reflecting mirror of your microscope or on the specimen directly. Many lamps are sold expressly for this purpose, but these are expensive and the simple homemade device shown in the photographs will answer your purpose.
To make such an illuminator, procure a metal-case, focusing-type flash light having a good reflector that will bring the lampfilament image to a point. Remove the batteries, if any, and cut the center part out of the case, leaving the two threaded ends and an inch or so of intervening metal. This process eliminates the switch also. Solder the two threaded portions together so that you have, in effect, an extremely short flash light case. Obtain a 6 to 8-volt bell-ringing transformer designed for 110-volt house lighting circuits. Run a band of metal around the flash light case and bolt it to one end of a pice of hard composition. Through the other end of the composition drill a hole and slip it over one of the secondary binding posts of the transformer. The transformer thus becomes the base use of the lamp.
Use a Mazda No. 40 lamp, the type intended for six-volt radio dial light installations, in the illuminator. This bulb, costing about ten cents, has a rated life of 1,000 hours on normal voltage. The transformer may produce a little more than six volts, which will shorten the life of the lamp a little, but will make up for it by giving a more brilliant light. You can, of course, operate the lamp on a six-volt storage battery instead of a transformer. The lamp holder, that used to be a flash light case, can be tilted up or down, to direct the light through the filters into the microscope mirror or on the microscope stage when opaque objects are being viewed.
Now that you have found how to make a ten-dollar microscope perform like one costing a small fortune, you are prepared to go visiting among some of the most expert workmen of the world and see, with the aid of your magic lenses, just how they carry on their delicate and deadly work. These workmen are the spiders whose webs you see every place, but whose beauty and wonders of craftsmanship you cannot appreciate without microscope.
Before you draw any rash conclusions about hte desirability of spiders as material for study and entertainment, consider two things about them: First, the spider does not deserve the bad reputation that most persons are inclined to give it. Perhaps you thing of a spider as something horrible, as a creature that will jump out and bite you with poisonous fangs at the first opportunity. The truth is that the spider doubtless is far more afraid of you than you are of it; and that only a few spiders possess a poison that is injurious to man. A spider merely seems horrible because you do not know him better.
Second, spiders have one big purpose in life - to capture and devour flies, along with other assorted insects. if ou keep this in mind, you can understand much of the work that the spider does, as well as appreciate some of his anatomy. it is to capture flies that a spider erecys a web that is a real engineering triumph. it is to kill flies that the spider ejects a poison through two fangs that the low powers of your microscope will reveal. it is to see flies that the spider has, on top of its head, a set of bright eyes. it is to enable the spider to bind a fly hopelessly in the twinkling of an eye, that it has a wonderful silk-making plant.
Soit's the old story of the spider and the fly that you are going to investigate. But what a different story it becomes under your lenses!
Before you start peering at spiders, inspect a few webs. if you can find one of those silk creations of an orb-weaving spider, that hang in your garden, you have material for an interesting hour at your microscope. Touch, carefully, one of the silk strands that run out from the center of the web to a point of support. Nothing happens, except that the spider crouching at the center may become excited. Now touch one of the strands that run across the radiating threads. it sticks to your finger.
Push two common pins into a piece of cork or soft wood, about an inch apart, and carefully pick some of the web threads, so that the silk is stretched between the upright pins. Try to get both the radiating and cross threads. Take this to your microscope, and focus it on one of the cross threads. At a hundred diameters, you will notice that the thread is not smooth as you had believed, but seems to be like a strong of beads. Step up the magnification to 300 or more diameters, and you will see that the silken stand really is covered with tiny beads, often arranged with large and small ones alternating. On the heavier radiating threads of the web, you find no beads.
You have discovered one of the secrets of the spider web. The droplets or beads on the cross threads are composed of a sticky substance secreted by the spider. Their purpose is to hold any fly that happens to blunder against the web. The spider, then, is the real inventor of fly paper.
Now that you have had a glimpse of the uncanny workmanship of the web-building spider, you are ready to carry your investigations closer to that master spinner. But you need not, as yet, capture a live spider. Look carefully about the webs in your cellar or garder, and you will find, in some of them, what appear at first glance to be dead spiders. These are not dead spiders, but are the cast-off skins of perfectly live and active individuals.
These skins make excellent material for study because, in casting them off, the spider let go with them may of the intimate details of its external make-up. The skins, for instance, show every detail of the fect with their interesting claws, the "chelicerae" with thei poison-ejecting fangs or clars, the remains of the silk spinerets, and even the hairs that cover the body and legs. The spider has, in shedding its skin, provided you with a better microscope specimen that you could prepare.
In handling this shed skin, you will have to exercise care because it is very light and fragile. A breath will blow it away, and careless handling with tweezers will crush the parts. if you want to preserve portions of the specimen, you can mount them in balsam, in built-up shellac cells. Be careful to eliminate all air bubbles from the hollow parts.
A spider has six pair of appendages or projections extending from its body. Beginning at the head and working backwards, you find first a pair of chelicerae, jawlike in appearance and in many species made up of two parts. These is a stout-looking, haircovered base of mandible with a curved claw-like fang at its end. This claw can be folded down agains the base like the blade of knife.
When a fly blunders into the web or is otherwise caught, the spider sinks its fangs into the victim's body, perhaps also wrapping it in a silken straight-jacket. Poison from glands situated inside the fore part of the spider's body is forced out through tiny ducts in the fangs, killing or stunning the fly. There are a few spiders whose poison is harmful to humans; but the chances are that if you avoid the Black Widow, you never will suffer a spider's bite that is poisonous enough to cause worry.
After the spider has poisoned its victim to insensibility or death, it proceeds to enjoy a feast. But it does not chew the fly and swallow it. insteadd, it brings into use the second pair of appendages, called by scientists the "pedipalpi." Their bases serve as jaws for pressing the food to extract the juices, much as you crush a lemon in a squeezer. The juices are taken in through a mouth which is connected to the sucking stomach. after the feast, the spider discards the crushed skeleton of the fly. You will find these remains in almost every web.
So far, you have disposed of two pairs of appendages. The four remaining pairs are the legs which, by their number, distinguish the spider family from that of the six-legged insects. The first pair of spider legs apparently serves much the same purpose as the antennae or feelers of insects.
Examine one of the legs under your microscope. You will find seven joints, with two or three claws at the end of the last segment. When there are three claws, two are arranged to form a pair, and the third is smaller and placed below the others. The large claws usually are equipped with teeth like a comb, and often tje small one is toothed. Web-spining spiders often have a number of accessory claws which are modified hairs that help them cling to the web.
Many species have, on the tips of their legs, groups of hairs that enable them to cling to smooth surfaces, probably because of a sticky fluid that is secreted. in addition to all this equipment, the spider's leg usually is covered with simple hairs. Some nature student believe that the presence of so many hairs on the legs of web-spinning spiders prevents them from falling, and web strands catching beneath the projecting gairs if the spider's foot slips.
Your investigations of the silk-manufacturing part of a spider's anatomy will begin with the spinnerets found at the rear of the abdomen, on the underside. You can see vestiges of these on the skin that the spider shed, but you can get a much clearer picture by using a recently killed specimen. SImply capture a web-spinning spider and drop it into bottle of alcohol for a few minutes.
The spinnerets, usually six in number, are arranged in three pairs. in general, they are covered with hair. They have hundreds of tiny tubes through which the silk-making fluid emerges from the silk glands.
Perhaps you wonder why there are so many silk tubes, and why an appartently singlre strand of web material really is the fusion of hundreds of tiny threads. The answer is: Speed! When a spider sidcovers a fly in its web, it rushes out and quickly wraps the victim in silken strands. The silk fluid, emerging through so many openings, hardens instantly upon exposue to air. Were there but one or at most a few openings, in comparatively large strands of silk would require more time to harden, and the spider could not spin its web rapidly enough to be of much help in building an insect trap or in binding its captives.
Now that you have been introduced to this master workman don't you feel a little more kindly towards it? Spider webs may be a nuisance in your parlor, but those in fields and gardens and wherever else they do not interfere with your activities are a real benefit because they kill flies.
It is partly in this way that your microscope earns its keep: it makes you axquainted with the little creatures with which you are forced to live, and helps you distinguish your friends from those that would harm you.