362 i:Ei\j. PIKE'S, JK., DESCRIPTIVE CATALOGUE. 



half Avay between the two lenses, for the purpose of 

 diminishing the spherical abberration. Each of these 

 eye-pieces fit within the upper end of the compound 

 body, and may be used as occasion requires. To the low- 

 er end of the compound body, which is somewhat conical, 

 are fitted, by fine screws, the object-glasses. With this 

 instrument there are usually three sets, having magnify- 

 ing powers of 100, 300, and 500 diameters, the first being 

 a combination of two achromatic lenses, and the two 

 others combinations of three achromatic lenses; these 

 can be used either singly or in combination, the latter 

 having the best effect. The coarse adjustment of the 

 object-glasses is made by sliding the compound body, A, 

 up or down, in a nicely-fitting tube fixed to its support, 

 while the fine adjustment is effected by the screw at the 

 milled-head, B, by which the support and the compound 

 body also are raised or depressed; the compound body 

 can not only be moved up and down in the tube, but 

 taken away for changing the powers or cleaning the 

 lenses. The condenser, K, or illuminating lens, for 

 opaque objects, is a plano-convex lens of short focus, set 

 in a brass frame, and supported by arms with moveable 

 joints to a brass foot ; by this arrangement the lens may 

 be set in any position required to receive the light, and 

 condense it on the object to be examined. There are 

 three diaphragms with this instrument ; they are black- 

 ened brass cells of about half an inch in diameter, one 

 having an aperture of a quarter of an inch, the others an 

 eighth and sixteenth ; they fit in the frame, H, from the 

 upper side of the stage-plate; their use is to modify the 

 rays reflected from the mirror, when required for the 

 object under examination, and to limit the pencil of 

 light allowed to fall thereon. The fluid cell is an oblong 

 brass frame, about an inch and a half long, and a quarter 

 of an inch deep, having a glass bottom cemented within ; 

 it is used for holding various fluids in their examination. 

 With the addition of a few test-objects, P, diseecting- 

 knife, II, point, S, and forceps, T, we have described 

 the usual parts and accompaniments of a good achromatic 

 microscope, in which great steadiness, accuracy of adjust- 

 ment, portability, and other valuable requisites, have been 

 successfully carried out. 



There remain several important appendages which 



