ACCESSORY APPARATUS. 19 



microscope, may be employed as a lens if the marginal 

 part is covered by a black-paper diaphragm. A very 

 convenient form of dissecting microscope is shown in 

 Fig. 3. This consists of a small compound micro- 

 scope furnished with 'an arrangement for reversing 

 the image, so that the object appears in its natural 

 position and not inverted. It is provided also with 

 a prism for reflecting the light in a direction conve- 

 nient for the eye, and is placed on a wooden stand 

 so constructed as to afford support to the arms of the 

 operator. The ordinary low power objectives may 

 be used with it. 



Besides the microscope, the student who is com- 

 mencing the practical study of histology will find it 

 necessary to be provided with the following simple 

 instruments and appliances : 



Glass-slides and cover-glasses (see Fig. 5). The mi- 

 croscopic slides are oblong slips of glass upon which 

 the object is placed. In this country they are always 

 cut of the uniform and convenient size of three inches 

 by one inch. The glass should be quite free from 

 flaws and specks. The cover-glasses can hardly be 

 too thin. It is true that when very delicate they are 

 likely to be broken in the cleaning unless great care 

 is exercised ; but then, on the other hand, many high 

 power objectives require to be focussed so close to the 

 object that a thick cover-glass cannot be used with- 

 out risk of crushing the tissue, and perhaps of scratch- 

 ing the objective. Asa rule, the chief difficulty in 

 cleaning them arises from the fact that their surfaces 

 are generally covered with a thin film, of grease or 

 other organic matter which it is almost impossible 

 to rub off. But this is easily got rid of by placing a 

 number of them together in a small glass beaker and 

 pouring a little strong nitric acid upon them. This 

 quickly destroys every trace of organic matter. The 

 acid is then poured off, and the cover-glasses are 

 thoroughly rinsed by allowing water to stream over 

 them from a tap for two or three minutes. They 

 are then to be kept in water ready for use, and need 



