10 



CHAPTER I. 



On Instruments and their Use: 

 The Microscope. 



THE microscope, as an instrument of power in 

 histological research, depends essentially in con- 

 struction, upon its conformity with the laws of light 

 and human vision. It has already been stated that 

 an idea of the size of an object is arrived at through 

 the size of the image focussed upon the retina, and 

 that these dimensions vary in proportion as the 

 object is brought near to, or removed farther from, 

 the eye. When the object is brought close to the 

 eye, its visual angle that is, the angle formed by 

 the crossing of the rays from the extreme points of 

 the object, is larger than when it is placed farther 

 off, and, consequently, the image on the retina is 

 larger also. If this principle were capable of un- 

 limited extension, it would obviously follow, that to 

 continue to increase the magnification of an object, 

 all one would have to do would be to bring the 

 object closer and closer to the eye. But there is 

 a limit to this natural power of microscopical vision 

 in the human subject, and the eye fails signally to 

 accomplish its office when the object is brought 



