rangements, as we have said, are more simple. Cheaper instruments, however, are made by English and American opticians, some of which are very fine.

Opticians divide microscopes into various classes, according to the perfection of their workmanship or the accessories supplied. The best first-class instruments have

FIG. 5.

Wenham's Prism for the Binocular Microscope.

a great variety of objectives and eye-glasses, mechanical stage with rack-work; a sub-stage with rack for carrying various illuminators: a stand of most solid construction; and every variety of apparatus to suit the want or wish of the observer. They are great luxuries, although not essential to perfect microscopic work. The second class, or students' microscopes, have less expensive stands, but equal optical powers, with first-class instruments. The

third or fourth classes of instruments are intended for popular and educational use, and are fitted not only with stands of more simple workmanship, but with cheaper lenses, although often very good. Some French achromatic objectives, adapted to this class, are suitable for all but the very finest work.

Binocular Microscopes.—The principle of the stereoscope has been applied to the microscope, so as to permit the use of both eyes. The use of such an instrument with low or medium powers is very satisfactory, but is less available with objectives stronger than one-half inch focus. There are two ways of accomplishing a stereoscopic effect in the microscope. The first and most common is by means of Wenham's prism (Fig. 5), placed above the objective, and made to slide so as to transform the binocular into a monocular microscope.

The second mode is to place an arrangement of prisms in the eye-piece, so as to refract one-half the image to the right and the other half to the left, which are viewed by the corresponding eyes. In either construction there is a provision made for the variable distance between the eyes of different observers. In the frontispiece is a representation of Zentmayer's grand American microscope, which will afford a good idea of the external appearance of a first-class binocular microscope. Students' and third-class microscopes, as before said, are less complicated and of more moderate cost. The mechanical and optical performance of Zentmayer's large instrument leaves scarcely anything to be desired. Instead of the more expensive rack-work stage, a simple form, originally invented by Dr. Keen, of Philadelphia, and copied by Nachet and others, is often employed. It consists of a rotating glass disk, to which is attached a spring, or a V-shaped pair of springs, armed with ivory knobs, which press upon a glass plate in the object-carrier. The motion is exceedingly smooth and effective.

Fig. 6 shows Collins's Harley binocular microscope, a good second class instrument.

Fig. 7 represents Beck's large compound miscroscope (monocular); and Fig. 8, Hartnack's small model microscope, with the body made to incline.

Fig. 9, Nachet's inverted microscope, invented by Dr. Lawrence Smith for chemical investigations.

CHAPTER III.

MICROSCOPIC ACCESSORIES.

In addition to the object-glasses, eye-glasses, mirror, and mechanical arrangement of the microscope, to which reference was made in the last chapter, several accessory instruments will be useful and even necessary for certain investigations.

The Diaphragm, for cutting off extraneous light when viewing transparent objects, is generally needed. In some German instruments it consists of a cylinder or tube, whose upper end is fitted with a series of disks having central openings of different sizes. The disk can be adjusted to variable distances from the object on the stage so as to vary its effects. English and American opticians prefer the rotary diaphragm, which is of circular form, perforated with holes of different sizes, and made to revolve under the stage. The gradual reduction of light can be accomplished by the cylinder diaphragm, since when it is pushed up so as to be near the stage it cuts off only a small part of the cone of rays sent upwards by the concave mirror, but, when drawn downwards, it cuts off more.

Collins's Graduating Diaphragm, which is made with four shutters, moving simultaneously by acting on a lever