8 VIEWS OP THE MICROSCOPIC WORLD. 



point ; and the image K L is as many times greater than F E, as the limit of dis- 

 tinct vision exceeds the principal foca. 



This is simply the rudimentary form of the compound microscope. As now con- 

 structed, both the eye-piece and the object-glass consist of a combination of lenses 

 by which the removal of very serious optical imperfections is effected. A full 

 description of these combinations, and of the nature of the errors they correct, can 

 be found in any good treatise on Optics. 



A very serviceable and well-constructed compound microscope, and one every 

 way adapted to the wants of the majority of microscopists, is "McAllister's Stu- 

 dent's Microscope," which is represented in the frontispiece. 



This instrument, when inclined, as shown in the engraving, stands twelve inches 

 high. The base B is of iron, lackered, with uprights to receive the axis upon 

 which the body inclines. The tube T is of brass, with extension draw-tube. 



The first adjustment for the focus is made by means of a delicate watch-chain 

 controlled by large milled head-screws, M M, on each side of the tube ; this adjust- 

 ment is extremely sensitive and exact, even with the highest powers of the instru- 

 ment. In addition to this there is a micrometer adjustment attached to the stage. 



The stage, S, is made of brass, and has brass springs to hold the object. A dia- 

 phragm plate, D, is placed below the stage, and the latter is also furnished with a 

 pair of forceps, F, which are very convenient for holding an object for examina- 

 tion. 



The mirrors, C P, concave on one side and plane on the other, are employed 

 for reflecting light upon the object which is placed over the opening, 0, in the stage. 

 The mirrors are so mounted that they can be turned in any direction. 



ACCESSORY APPARATUS. 



MICROMETERS. In examining objects with the microscope it is often desirable to 

 ascertain their exact dimensions. Measuring-instruments, called micrometers, are 

 therefore sometimes applied to the object itself, and sometimes to the magnified 

 image. When applied to the object the stage micrometer is employed, which con- 

 sists of a slip of glass upon which parallel lines are evenly ruled with the point of 

 a diamond, so close together that 2,500, and in some cases 50,000, occupy only the 

 space of one inch. This apparatus is placed upon the stage, and the object exam- 

 ined measured by it. Of the second kind is Jackson's micrometer slide, which is 

 so arranged in the eye-piece of the microscope that it can be brought over the mag- 

 nified image by means of a screw. 



THE DIAPHRAGM. To the under surface of the stage is generally affixed a circu- 

 lar plate of metal pierced with holes of various diameters ; this is called a diaphragm. 

 It is used to modify the amount of light thrown by the mirror through the orifice 

 in the stage upon the object. By turning the plate the holes can be brought suc- 

 t-ly under the opening in the stage. 



THE ILLUMINATION OF OPAQUE OBJECTS. This is effected in several ways : 1st. By 

 condensing the light upon the object by means of a large, simple, plano-convex lens r 

 called a bull's-eye condenser. 2d. By concentrating light upon the object by means 

 of a metallic mirror fitted to the side of the instrument. 3d. By causing the rays 

 of light reflected from the mirror, and passing round the circumference of the object, 

 to impinge upon a concave annular reflector, called a lieberkuhn. Thia mirror is adapt- 



