Renna 
ELEMENTS OF APPLIED 
MICROSCOPY. 
CHAPTER I. 
FUNCTION AND PARTS OF THE MICROSCOPE. 
1. Normal Vision.—In the ordinary act of vision at least 
two distinct processes may be distinguished, the formation 
upon the retina (K, Fig. 1) 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 
