instrument, slip it on the end of the microscope in the place of the eye-piece, and place object No. 14, human blood, on the stage with a one or two inch objective. The spectrum will now be seen to be traversed by two marked black lines in the red, which form an excellent test for the presence of blood. Their position may be measured with the scale, by attaching the latter to the side of the eye-piece, and adjusting the prism so that the spectrum for one half its breadth shall be traversed by strongly marked black bands. Other objects, such as nitrate of didymium, permanganate of potash and aniline violet, may be observed in a similar manner. Care should be taken to make all the light pass through the object, which is generally best accomplished by placing a cardboard diaphragm with a small hole in it, on the stage under the object. Liquids are placed in glass tubes or cells, which may be closed hermetically.

15. Test-Objects. The principal efforts of microscope makers are now directed towards the objectives, since it is by perfecting them that the greatest improvements are to be expected. The best method of judging of the excellence of an objective, or of comparing those of different makers, is by trying them on a number of objects called test-objects, some parts of which can be seen only with difficulty. To obtain the best results great skill is needed, especially in arranging the illumination, and it must not be forgotten that some objectives give the best results with one class of objects, others with another. For instance, some with a large angular aperture, give fine effects with objects requiring a very oblique illumination, but are not suited to those of considerable thickness, requiring great depth of focus.

When an objective is perfectly corrected for chromatic aberration, and a plate of thin glass is interposed between it and the object, a new correction for color becomes necessary, in amount depending on the thickness of the glass. This is commonly effected by varying the distance of the front lens from the other two, which is accomplished by turning a milled head near the end of the objective. A divided circle and index serve to mark the position, which will of course vary with each different object, according to the thickness of the covering glass. To make this correction, adjust the objective for an uncovered object, that is,

set the index at zero and focus it on the object. Then turn the milled head until the dust on the upper surface of the covering glass is in focus, when the proper correction will have been applied. Focussing again on the object, the latter will be more sharply defined than before. The correction for covering glass, as it is called, must be applied to all objectives of higher power than inch, to get the best effects, especially when they have a large angular aperture. Instead of moving the front lens, it is better to have it fixed, and to have the other two movable, as all danger of scratching or breaking the objective and object by bringing them in contact, is thus avoided.

Try some of the higher power objectives with the test-objects No. 15. One of the most common tests for defining power is the marking of the scales of the wood-flea (Podura plumbea), which are covered with delicate epithelial scales, like the tiles of a roof. Try also the hair of the Indian bat, and of the larva of the Dermestes. Some of the Diatoms described above, form excellent test-objects. The valves of the genus Pleurosigma are covered with fine markings, which form an excellent test for separating or penetrating power. For instance, the three species, formosum, hippocampus and angulatum, form a series of increasing difficulty, well adapted to test objectives of ordinary power. The marking of the first and third are apparently covered by three series of fine parallel lines, dividing the surface into hexagons, and of hippocampus by two series, forming squares, but in reality probably due to a multitude of very minute hemispheres with which the surface is covered. The same effect may be seen on an enlarged scale, in a common form of book-cover. Probably the best test of this kind is a plate of glass with very fine lines ruled on it. M. Nobert of Griefswold has made such plates with a series of bands formed of lines at various intervals up to a 112,000th of an inch.

80. PREPARATION OF OBJECTS.

Apparatus. A microscope with objectives and eye-pieces, several vials containing the substances to be examined, a number of glass slips three inches long and an inch wide, some of which have concave centres, that is, a concavity ground out on one side, and some circles of very thin glass.

Experiment. To examine a liquid under the microscope, dip a glass rod or tube into it, and place a small drop on one of the glass slides. Cover it with a circle of very thin glass, which will be held in place by capillarity, and wipe off the superfluous liquid carefully. A concavity is commonly ground in the centre of the slide to hold more liquid, and to keep the cover in place. Examine the following objects in this way, describe and sketch them, and compare their appearance with that given in the works on the Microscope, referred to in the last experiment. A drop of vinegar viewed with a low power, is seen to be full of eels in active motion. Milk contains multitudes of oil globules, which when united form butter, and organic matter whose appearance furnishes an excellent test of its purity. Blood is a curious object under the microscope. It is most readily obtained from the finger just below the nail. With a quarter-inch objective, it is seen to consist of a clear liquid or serum, in which a vast number of blood-corpuscles are floating. These are circular disks, thicker around the edge, and interspersed with larger white globules. In its natural state the blood is too thick to be conveniently observed, the corpuscles overlap, and soon begin to shrivel up, as the blood dries. If diluted with water osmotic action ensues; they swell up and sometimes burst. Salt water is therefore preferable, or better still, the serum or liquid portion which separates from the clot when blood coagulates.

Powders are sometimes viewed dry, but generally it is better to wet them, as they are thus rendered more transparent. Place a very minute quantity of starch on a slide, add a drop of water, and cover with a piece of thin glass. Viewed by polarized light, each grain is seen to contain a black cross, which changes to white on rotating the analyzer. This cross is characteristic of starch, and often serves to detect its presence. It is best seen in the larger grains, as those of potato starch, and assumes brilliant colors if a plate of selenite is interposed. The adulterations of coffee, cocoa, etc., are readily detected by examining them in powder under the microscope.

It is often necessary to pick up small objects under water, or to capture a minute animal without injuring it. A good example of this kind is the little Cyclops, often found in great numbers in common pond water, especially in the spring. Collecting the

water in a white porcelain vessel, as a large evaporating dish, a close examination will often reveal dozens of them. Their number may also be increased by filtering a considerable quantity of the water through a cloth, which retains them, and from which they are easily washed into the dish. To place one on the slide, take a small glass tube about half a foot long, close one end by the finger, and immerse the other in the water. Bring it near the Cyclops and suddenly remove the finger, when the water will rush in, carrying the animal with it. Replacing the finger, the tube may be removed, the water allowed to escape a drop at a time, and the Cyclops finally deposited on the slide. Instead of a slide with concave centre, it is better for so large an object as this, to use an Animalcule-Cage. This consists of a small circle of glass, on which a drop of water containing the object is laid, and the cover pressed down upon it by means of a brass ring, so as to leave a space of any desired degree of thickness. Delicate objects are thus protected from injury by crushing. A wonderful variety of animalcule and of fungoid growths may be found in stagnant water, or sour flour-paste, in fact in almost any decaying animal or vegetable matter.

Minute air-bubbles are often found in various objects. To become familiar with their appearance, examine a drop of soap-suds, or gum-water containing them. They look like black, highly polished, metallic balls, with a broad, dark outline, and bright centre.

The formation of crystals is readily watched under the microscope by placing a drop of the hot saturated solution on a slide, and allowing it to cool. Try in this way sugar, phosphate of soda, and oxalate of ammonia, first using ordinary, and then polarized light.

A most instructive experiment is to watch the circulation of the blood in the foot of a frog. The animal is first rendered insensible by means of ether or chloroform, then put in a linen bag and well wet with water. Draw one of the hind legs out of the bag and tie it down upon the slide, supporting the frog on a piece of wood or frog-plate. Tie threads to three of the toes, so as to stretch the membranes between them, and on examining it with a half-inch objective the blood corpuscles will readily be seen passing from the arteries through the capillaries to the veins. By putting alco

hol or salt on the foot, all the phenomena of inflammation and its cure may be observed. The black spots distributed over the membrane are due to the pigment. The circulation may also be observed in the tail of a stickle-back, or other small fish, or of a tadpole. The latter animal, when very small, forms a beautiful object with a low power and binocular microscope, as it is sufficiently transparent to render visible the action of the heart, and other internal organs. The effect is also improved by keeping the tadpole for some time previously without food.

Another interesting experiment is to watch the ciliary action, which in many of the lower animals takes the place of the circulation. Cilia consist of minute hairs, which vibrate rapidly back and forth, and thus establish currents in the liquid in contact with them. They may be seen by scraping a little of the mucus from the roof of the mouth of a frog, or better, from the gills of an oyster or mussel.

Most solid substances, like wood or bone, are best seen in thin sections, which are made as will be described in the next Experiment. Fine filaments, as silk, wool or hair, are viewed by transmitted light, and generally give better effects when wet with water or oil. Some solids, especially when highly colored, are best seen as opaque objects, with a condenser or lieberkuhn.

81. MOUNTING OBJECTS.

Apparatus. Boxes may be obtained containing all the apparatus needed for mounting objects, such as glass slips, thin glass covers, Canada balsam, gold size, a stand on which slides may be heated, a whirling table for making cells, section-cutting apparatus, and other devices which will be described below.

Experiment. Objects are mounted in various ways, according to their size, and whether they are best seen dry, or immersed in some liquid. They are protected by a circular piece of glass, made very thin on account of the short working distance of highpower objectives. These circles are cut from a sheet of thin glass with an instrument like a very small beam-compass, the point which serves as a centre, being replaced by a flat disk, and the pencil, by a diamond. Only a faint scratch is needed, but some skill is required, or many of them will be broken.