ELEMENTS OF APPLIED 

 MICROSCOPY, 



CHAPTER I. 



FUNCTION AND PARTS OF THE MICROSCOPE. 



I. Normal Vision. — ^In the ordinary act of vision at least 

 two distinct processes may be distinguished, the formation 

 upon the retina (K, Fig. i) of a minute image of the object 

 seen, and the transformation, by the optic nerves and 

 their end organs, of the energy of the light-waves thus 

 thrown upon the retinal surface into that form of nervous 

 activity which we call sensation. The first process fol- 

 lows the simple laws of Optics, while the second is con- 

 trolled by the far more complex chemical and physical 

 conditions which furnish the subject-matter for the sci- 

 ence of Physiology. The sensation experienced is di- 

 rectly related to the image lying upon the retina; that 

 is, to an area of special illumination whose parts corre- 

 spond to those of some object outside, from which the 

 illumination is derived. The mind has learned, by 

 experience, to interpret a certain image upon the retina 



