A mirror is placed under the stag,e for reflecting the light through the object unde,r observation. This method of illumination by transmitted light is used when the object is transparent. When opaque, light is reflected on the object by a bull's-eye lees, called a condenser. The best instruments are supplied with six or seven object-glasses, varyiug in magnifying power from 20 to 2,500 diameters. The eye-pieces supplied are three in number, each of which consists of two plano-convex lenses, between which a stop or diaphragm is placed, half-way between the two lenses. As the magnifying power of a compound microscope depends on the product of the magnifying powers of the object glass and the eye-piece, it follows that its power may be increased or diminished by a change in either or both of these glasses. In the mechanical arrane.emeuts, it is of importance to have the instrument so constructed, that while every facil ity is afforded for making obsenations and easy- means of adjustment, there should also be great steadiness, without which, indeed, no satisfactory results will be obtained.
These ends arc achieved in various ways, of which Fig. 6 is one of the simplest: a, brass stand, supported on three feet; b, mirror, supported on trunnions; c, diaphragm, pierced with circular holes of various sizes, to regulate the admission to the object of reflected light from the mirror; d, stage-plate, on which the object is placed; e, screw, with milled head for fine adjustment-if, the object-glass, or objective; g, brass tube in which the body of the instrument is moved, so as to effect the coarse adjustment; lt, the eye-piece, or ocular.
The microscope has now become so important an instru ment in education, that almost every department of science in which it can be employed has a microscope suited to its par ticular kiud of work, and a special tmatise explaining aud illustrating its use; and many branches of science have instru ments peculiarly their own. Thus, chemists, anatomists, zoolog,ists, etc., have each an instrument which they value as being peculiarly adapted for their special fields of inquiry and observation. From this instrument the chemist, and natural philosophers generally, have derived great assistauce in study ing the different kinds of crystals; ter, by means of it, they can not only observe and recognize the great variety of forms that exist, but at any moment, and with liule trouble, they may witness the process of crystalllzation, and leisurely study it. Those sciences in which it is most used, and for which it has done most, are anatomy, physiology, botany, zoology, medicine, mineralog,y, and geology. In the practice of medicine all medical men who aim at a scientific treatment of disease have fully recoguized how useful it ha.s been as an agent in diagnosis, more especially in diseases of the kidneys, In the detection of crime and the vindication of innocence it is no less useful, as 17 means of it we can with certainty determine whether a suspicious stain, found, for instance, on the clothing of an individual charged with murder has been caused by blood or by another coloring-matter. In like manner we can determthe whether hair found in similar circumstances belongs to a human being or not It has also enabled us to distinguish the difference existing between substances that have a similar chemical reaction (e.g., the various kinds of starch, as flour, potato, sago, etc.), and thus we are
provided with an agent quick in detecting adulteration.
A few hints to amateur observers may not be out of place here. In choosing an instniment, the simpler it is the better. The essential point to attend to is to have good glasses, which ate tested by their power of showing some very minute markings, such as we find on diatoms, The circumference of the field of view should not be tinged with color, and the definition should be as good at the edge as at the center. The beginner should use low powers in preference to high ones. The best light is that reflected from a white cloud duriug the day. Artificial light should, if possible, be avoided. The table must be steady on which the microscope is placed, and when not in use the instrument should be covered bv means of a glass shade. The observer also requires a few oblong glass slides, and a few circles of thin glass, called covering-glasses, to lay over the preparation under examination. For making. sections, dissecting, and the various manipulatory operations attending the use of the micfoscope, lie requires, moreover, a pair of forceps, a knife, or, perhaps better, a razor ground tlat on the one side, a few needles fixed in handles. and two or three hair-pencils. So equipped, the observer is able to begin examinations of texture at once with pleasure and advantage. Begin with simple objects, such as pollen and thie slices of the cuticle of flowers. mosses, and different kinds of starch, such as tous' le mols, buck yam, cyeas, arrow-root, etc., and notice particularly their different characters. Make as thin a seetiou as possible, place it on the center of the slide, mid allow a drop of water to fall on it from the end of the handle of the needle. Then allow the covering-glass to fall gently on it—obliquely, so as to press out any small bubbles of air. He shonkl also have a few bottles containing " reagents," sucu as dilute acetic acid (equal parts of pyrolig neous acid and water) and liquor potasste, By mean; of these reagents, peculiarities of structure may ofteu be observed.
Microscopes vary much in price, from 5s. to upwards of £100. A good serviceable dissection. simple microscope may be had from any philosophical-instrument maker for from 9s.c'to 15s. Compound microscopes are more expensive, but a wonderfully good instrument for bet„rinners can be had at 30s. It has one eye-gla.ss and three object-glasses, and man.nifies front TO to 200 diameters. If a superior instrument is wished--one suited for mos-t purposes of observation and research—any- one of the following will be found weIl worth the price- The microscope of IIartnack, with a joint, so that it may be inclined at any angle, has two eye-pieces, two object-glasses, magnifies from 50 to 450 diameters, and costs about £7; Nachet's microscope has three eye-pieces, three object glasses, tnagnifies from 50 to 750 diameters, and costs £10; Smith and Beck's educa tional microscope has two eye-pieces, two object-glasses, magnifies from 50 to 350 diam eters, and costs £10; Ross supplies microscopes front £5 to £100, with various number of g-lasses.
For a more complete account of the different kinds of microscopes, and the various purposes to which they are applied, see Queltett On the Microscope (1855); Carpenter On the Microscope (1862); Hogg On the Microscope (1855); and How to work with, the Micro scape (1864), by Beale.