2 METHODS OF MICROSCOPICAL EESEAECH. 



loses its power of accommodation in exerting the 

 strain required to render the crystalline lens suffi- 

 ciently convex to focus the image of the object 

 upon the retina; about two inches from the eye 

 this action becomes impossible. Within the limits 

 of accommodation megascopical characters only can 

 be appreciated. If the eye were capable of altering 

 the curvature of its crystalline lens indefinitely, its 

 power of vision would become illimitable, and tele- 

 scopes and microscopes and other such instruments 

 would be found unnecessary. But the bounds of 

 natural limitation can be conquered by artificial 

 means, and the interposition of a sufficiently convex 

 lens between the near object and the eye alters the 

 direction of the rays of light which proceed from 

 the object so as to bring them within the scope of 

 natural vision, and herein lies the theory on which 

 the microscope has been constructed. The nearer 

 an object approaches the eye, the greater does its 

 visual angle (or angle produced by the intersection 

 of rays, or straight lines, from the extreme points 

 of the object) become, and, consequently, a larger 

 image is focussed upon the retina. 



Optical instruments, therefore, are required on 

 the very threshold of our " Studies " to enable us 

 to enter the domains of histology and microscopical 

 investigation, and they may be employed directly in 

 a multitude of instances when the objects are of 

 minute size. Equally numerous, however, are the 

 substances and organisms which cannot be thus 

 directly examined, but which require to be sub- 

 jected to special processes and manipulation in 

 order to render them suitable for microscopical 

 examination. In the inorganic kingdom some 



