338 SCIENTIFIC APPARATUS. 



APPARATUS USED IN PREPARING MICROSCOPICAL OBJECTS. 



Many of the small tools used in preparing microscopical 

 objects are of such a general nature that they need not be par- 

 ticularised. Attention may, however, be called to the various 

 forms of apparatus used for cutting thin sections of wood, and of 

 similar substances, that can be sliced with a sharp knife, and to 

 those in which arrangements are made for freezing soft, wet 

 animal tissues, and cutting them in a frozen state. Small lathes 

 and wheels are used for cutting, grinding, and polishing sections 

 of rocks, minerals, fossils, shells, and other similar more or less 

 hard and brittle substances. These may save time and material, 

 but are by no means essential, since first-rate sections may be 

 prepared by holding the objects in the hand, and grinding them 

 down on flat metallic plates, with somewhat coarse and finer 

 emery, and finishing them off on flat stones of various quality. 



SPECTRUM APPARATUS. 



The more accurate study, by means of spectrum analysis, of 

 the exact nature of the light transmitted by, or reflected from, 

 coloured substances, has led to the introduction of a number of 

 new forms of accessory apparatus. The analysis of the light is 

 effected either by means of a special spectrum eye-piece, having 

 the slit at the focal point of an achromatic eyeglass, over which 

 is placed a compound direct-vision prism, or by means of appa- 

 ratus having the slit at the focal point of the object-glass. The 

 former class of apparatus is far the most suitable for the examina- 

 tion of very minute objects with high powers, since they are in 

 focus along with the spectrum, whereas a form of apparatus, placed 

 below a special object-glass of long focal length, is far more con- 

 venient in studying with a binocular microscope the spectra ot 

 solutions held in small glass cells, or of any objects not less than 

 one-eighth of an inch in diameter, which need not be exactly in 



