CHAPTER XII.

THE USE OF THE MICROSCOPE IN THE STUDY OF PHYSIOLOGY.

[Sidenote: 1. The will of the Creator, by what obeyed? The power of a muscle? Amount of duty performed by the liver?]

1. THE LAW OF THE TISSUES.--The will of an infinite Creator is obeyed by atoms as well as by worlds. He has seen fit to commit all the functions of life to structures or tissues so small as to be invisible to the naked eye.

A muscle, for example, as we have already learned, is composed of innumerable filaments, visible only by the aid of the microscope; and the power of the muscular ma.s.s is but the sum of the contractile power of the filaments which enter into its composition. Again, each cell of the liver, invisible to una.s.sisted sight, is a secreting organ, and the liver performs as much duty as the sum of these minute organs renders possible and no more.

[Sidenote: 2. Necessity for using the microscope? The advantages gained by its use?]

2. THE NECESSITY OF THE MICROSCOPE.--If, therefore, we would know the real structure of the human body, we must make use of the microscope. We are not at liberty either to use it or not; we _must_ have recourse to it in order to obtain a real knowledge of the human body. Our eyes are constructed for the common offices of life, to provide for our wants and guard us from the ordinary sources of danger; but by arming them with _lenses_, the real structure of plants and animals is revealed to our intelligence; and enemies, otherwise invisible, that lie in wait in the air we breathe, and in our daily food and drink, to destroy life, are guarded against.

[Sidenote: 3. What are convex lenses? Kind of lenses used in microscopes?

Experiment? Picture thrown upon the eye? Derivation of the word microscope?]

3. CONVEX LENSES, or magnifying gla.s.ses, are disks of gla.s.s or other transparent substance, which have the {237} property of picturing upon the retina of the eye an image of an object larger than the image produced there without their aid. The gla.s.ses used in microscopes are either double convex lenses (_a_) or plano-convex lenses (_b_). If a double convex lens or a plano-convex one be placed over a hole in the shutter of a darkened room, or over the key-hole of a door, and a piece of paper be held at a proper distance, a picture of all objects in front of the lens will be thrown on the paper, as in the camera-obscura or the magic-lantern. Now, in the same manner, a lens throws a picture of objects to which it is directed on the retina of the eye, and when that picture is larger than the image made in the eye by the object, without the aid of the lens, it is magnified, or the lens has served as a _microscope_, so called, from its use in seeing small objects, from _mikros_, small, and _skopeo_, to see.

[Ill.u.s.tration]

[Sidenote: 4. Kinds of microscope? What are simple microscopes?]

4. DIFFERENT KINDS OF MICROSCOPES.--Microscopes are either _simple_ or _compound_. The gla.s.ses of magnifying spectacles, like those commonly used by aged persons, are simple microscopes. Magnifying gla.s.ses, mounted in frames such as are for sale by opticians and others, for the detection of counterfeit money, are simple microscopes, and are useful in studying the coa.r.s.er structure of plants and animals.

[Sidenote: 5. Construction of the most powerful simple microscopes? In practice? A doublet? Triplet? Why are compound microscopes superior to simple ones?]

5. The most powerful simple microscopes are made by melting in a flame a thread of spun gla.s.s, so as to form a {238} minute globule or bead, which, when set in a piece of metal and used to examine objects on a plate of gla.s.s held up to the light, gives a high magnifying power. In practice, however, it is found better to use several magnifying gla.s.ses of moderate power, than a simple lens alone of high power. A combination of two lenses is called a _doublet_--of three, a _triplet_. All _simple_ microscopes throw an enlarged image of the object upon the retina. _Compound_ microscopes are so constructed that the enlarged image of an object is again magnified by a second lens, and hence their magnifying power is vastly superior to that of simple microscopes.

[Sidenote: 6. Explain, by means of the diagram, the action of the compound microscope.]

6. The accompanying diagrams will explain the action of the compound microscope compared with that of the simple microscope. In Fig. 58, which represents the working of the simple microscope, the rays from the object (_a b_), pa.s.sing through the lens (L), form an image (_a' b'_) in the retina of the eye (E), and as all images are inverted in the eye, the object is seen as all other objects are, and appears erect. In Fig. 59 is seen the action of the compound microscope. An inverted image (_a' b'_) of the object (_a b_) is magnified by the second lens (L'), and an erect image is thrown upon the retina, which, as all other objects seen erect with the naked eye are inverted, gives to the image a contrary direction, or inverts it to the mind.

[Ill.u.s.tration: FIG. 58.--SIMPLE MICROSCOPE.]

{239}

[Sidenote: 7. Portions, in a compound microscope? The gla.s.ses?]

7. A COMPOUND MICROSCOPE consists of two portions: the optical portion, or the lenses, and the mechanical portion, or the instrument which bears the lenses. The gla.s.ses of a compound microscope are two: the _object-gla.s.s_ (D), Fig. 60, and the lower lens of Fig. 59, and the _ocular_ or _eye-piece_ (A), Fig. 60, and the upper piece of Fig. 59. Both the object-gla.s.s and the eye-piece may, and usually do, consist of more than one lens, for, as previously mentioned, better results are obtained by a combination of lenses of moderate power than by single lenses of high power and great curvature.

[Sidenote: 8. How to choose a microscope? How to use it?]

8. HOW TO CHOOSE AND USE A MICROSCOPE.--No attractiveness in the mechanical part of a microscope can compensate for inferior lenses; and the very first consideration in the choice of an instrument should be the excellence of the optical part of the instrument. In the use of the instrument, care should be exercised to keep the lenses clean, free from dust, not to press the object-gla.s.s upon the object under observation, and not to wet it in the water in which most objects are examined. A good microscope requires its own table; and when not in use should be covered by a bell gla.s.s or a clean linen cloth.

[Ill.u.s.tration: FIG. 59. COMPOUND MICROSCOPE.]

{240} [Ill.u.s.tration: FIG. 60.

A, Eye-piece. B, Body. C, Collar. D, Object-gla.s.s. E, Stage. F, Hinge.

G, Mirror. H, Stand.]

[Sidenote: 9. The characteristics of the best instrument? What special requisites should be insisted upon? Why, as to a horizontal stage?]

9. The mechanical portion of the instrument varies greatly in different instruments. That one is the best which is simplest, the most solid and easily managed. The stage (E), upon which the object is placed, should not be movable: it should be solid and firm. The screw by which the focal distance is adjusted, and which {241} is in constant use, should be so placed that it can be worked by the hand resting on the table: otherwise fatigue is soon induced. The direction of the tube carrying the gla.s.ses should be perpendicular, and the stage therefore horizontal. Most objects in human anatomy are examined in water or in other liquids, or they are themselves liquids; hence an oblique stage is often inconvenient.

[Ill.u.s.tration: FIG. 61.]

{242}

[Sidenote: 10. Slides? Covers, square and circular? How kept?]

10. ADDITIONAL APPARATUS.--As almost all objects in human anatomy are examined by transmitted light thrown up from the mirror (G, Fig. 60) beneath the stage through the object to the eye, they must be placed upon strips of clear gla.s.s about three inches long and one inch wide, commonly called "slides." These should be procured with the microscope. Again, most objects seen with high powers require to be covered with a thin plate of gla.s.s, very properly called a "cover," that the moisture of the specimen may not tarnish the object-gla.s.s. Square or circular covers of very thin gla.s.s are therefore provided; and a good supply of these should be always on hand. These gla.s.ses should be kept in a covered dish filled with a mixture of alcohol and water. Simple water will not remove the fatty matter which exists in all animal tissues, and, therefore, the gla.s.ses cannot be thoroughly cleaned with it alone.

[Sidenote: 11. Cleaning the gla.s.ses? Knives, scissors, etc.? Various liquids?]

11. When gla.s.ses are required for use, they should be removed from the liquid and wiped clean and dry with a soft linen handkerchief. Delicate knives, scissors, needles mounted in handles, forceps, pipettes or little tubes for taking up water, should be obtained; these are essential to all microscopical study. The table should be supplied with gla.s.s-stoppered bottles containing the various liquids ordinarily used in the study of physiology. Thus, tincture of iodine is indispensable in studying vegetable structure, acetic acid in the study of animal tissues; and other articles will have to be added from time to time, as your progress in study demands them. {243}

[Sidenote: 12. Bodies, in air and water? The examination of starch?]

12. PRELIMINARY STUDIES.--In order to prepare the way for the study of any department of science with the aid of the microscope--for the microscope is but an eye, and can be turned in almost any direction for purposes of investigation--it is necessary to become acquainted with the many objects which are liable to complicate the examination of particular structures.

Both air and water are full of floating bodies, and the most common of these should first occupy the attention. In the city, particles of starch are always floating in the air. Take a very minute portion of wheat flour, place it in the middle of a clean gla.s.s "slide," drop upon it a drop of pure water, cover it with a plate of thin gla.s.s, and examine it with a power of from one hundred to six hundred diameters. It will be found to be composed of minute grains or granules, the largest of which are made up of coats or layers, like an onion, arranged around a central spot called the _hilum_.

[Sidenote: 13. The examination with solution of iodine? Advice respecting other articles?]

13. Make another preparation in the same manner, and, after adding the water and before covering with the thin gla.s.s cover, add a small drop of a solution of iodine. Now, upon examining the specimen, every grain will be seen to be of a beautiful deep blue color. After thus studying wheat starch, the starch of Indian corn, of arrowroot, and of various grains should be examined in like manner, and their resemblances and differences noted. The granules of potato-starch are as distinctly marked as any. (See Fig. 15, page 61.)

[Sidenote: 14. Directions for examining cotton and other fibres? Vegetable hairs?]

14. Fibres of cotton, lint, and wool are liable to be found in every specimen prepared for microscopical examination. In order to study these, any cotton, woollen, or linen fabric, or garment, may be sc.r.a.ped, and the sc.r.a.pings placed on a piece of gla.s.s moistened with water, covered with the thin gla.s.s plate or cover as before, and {244} examined with the same magnifying power, namely, from one hundred to six hundred diameters.

Vegetable hairs or down are constantly floating in air and water. These are of very various forms, are simple or grouped, and form very interesting objects of study. They are readily procured from the epidermis or outer membrane of the leaves or stems of plants, by section with a delicate knife.

[Sidenote: 15. Directions for examining various tissues? Down of moths, and other structures?]

15. The tissues of plants, epidermis, ducts, and woody fibres are constantly found in microscopic preparations. They may be studied in delicate sections made with a sharp knife, or by tearing vegetable tissues apart with needles. The down of moths, the hairs of different animals, the fibres of paper, the most common animalcules in water, the dust of shelves, and generally the structures found in all vegetable and animal substances by which we are surrounded, should be studied as a preliminary to any special line of microscopical investigation.

[Sidenote: 16. Directions for examining a drop of blood?]

16. THE STUDY OF HUMAN TISSUES.--When this has been done and familiarity with the use of the instrument has been obtained, proceed to the study of the human body, for human physiology is our subject. If the end of the finger be p.r.i.c.ked with a pin, a drop of blood may be procured for examination. Place this on one of the gla.s.s slides, cover it with a thin piece of gla.s.s, press down the cover so as to make a thin layer, and then examine with the magnifying power just mentioned. Do not add water, for that will cause the blood corpuscles to disappear. If the drop of blood is placed under the microscope at once after being drawn from the finger, most interesting phenomena will be observed. The red corpuscles will be seen to arrange themselves in rows, like piles of coin, while the blood is coagulating. The spherical, white corpuscles will {245} be left out of the rows of red disks, and, if the highest power be used, will be seen to change their shape constantly.

[Sidenote: 17. Examination of the scales of the mouth? Dandruff?]

17. If you sc.r.a.pe with a dull knife the inside of the cheek, the flattened scales of "pavement epithelium," or of the insensible covering which, a.n.a.logous to the scarf-skin on the outer surface of the body, lines the cavities of its interior, may be readily studied. They have the appearance of transparent tiles, each enclosing a round or oval body, called its nucleus. Dandruff and the sc.r.a.pings from the skin of the body are composed of scales like those of the mouth, but they differ somewhat in being hardened by h.o.r.n.y matter, and in having a very faint central body or nucleus.

[Sidenote: 18. In what, as respects the tissues, do the warm blooded animals differ? Statement of Milne Edwards?]

18. THE TISSUES OF THE INFERIOR ANIMALS.--The warm-blooded animals do not differ in the tissues, or microscopic structures, that compose them, but only in the amount and arrangement of these tissues. Milne Edwards says these tissues "do not differ much in different animals, but their mode of a.s.sociation varies; and it is chiefly by reason of the differences in the combination of these a.s.sociations in various degrees, that each species possesses the anatomical properties and characters which are peculiar to it."

[Sidenote: 19. How to procure materials for the study of the tissues of man?]