MODERN METHODS OF EXAMINATION. 59 



dition, and many require to be dissected or preserved in 

 fluid. This has much to do with the appearance of the 

 object in the microscope. If fibres or cells are imbedded 

 in connective tissue or in' fluids, of which the refractive 

 power is the same as their own, they cannot be perceived 

 even with the best glasses, and artificial means must be 

 resorted to that they may become visible. The refractive 

 power of different media causes different appearances. 

 Thus a glass rod lying in water is easily seen, but in 

 Canada balsam, whose refractive power is nearly the same 

 as glass, it is barely seen as a flat band, and in the more 

 highly refractive anise oil it presents the appearance of a 

 cavity in the oil. 



During life the cavities and fissures in animal tissues, 

 in consequence of the different refractive power of their 

 contents and the change which takes place soon after 

 death, exhibit a sharpness and softness of outline which 

 is seldom seen in preparations. 



There are two methods of microscopic investigation or 

 of preparation preliminary to direct observation: 1. Me- 

 chanical, for the separation and isolation of the elemen- 

 tary parts. 2. Chemical, which dissolve the connecting 

 material, or act on it differently than on other elements. 



For minute dissection a great variety of instruments 

 have been proposed, but by practiced hands more can be 

 accomplished in shorter time by simple means than with 

 complicated ones. Two or three scalpels, or small ana- 

 tomical knives, a pair of small scissors, such as is used in 

 operations on the eye, and fine-pointed forceps, will be 

 found useful. But the most serviceable instruments are 

 dissecting-needles, such as the microscopist may make for 

 himself. A common sewing-needle, with the eye end 

 thrust into a cedar stick aboiit three inches long and one- 

 fourth of an inch diameter, will answer the purpose. The 

 point should not project so far as to spring, and if desired, 

 a cutting edge can be given to it by a hone. 



