THE ORGAN OP VISION. 17 



The Organ of Vision. Before passing to the con- 

 sideration of the formation of images by the human 

 eye, let me say that owing to want of space, I am unable 

 to enter as fully as I could wish on the consideration of 

 the microscope as an optical instrument. Those of my 

 readers, however, who desire to become better acquainted 

 with the physical optics of the instrument, will do well 

 to consult one or more of the numerous standard works 

 devoted to this special branch of physics, or, what 

 may even be more profitable, secure a book specially 

 devoted to it. I can confidently recommend for profit- 

 able study the translation (already indicated) of 

 Nageli and Schwendener, " Theory and Practice of 

 the Microscope," by the indefatigable Secretary of the 

 Royal Microscopical Society, Mr. Frank Crisp. 



The formation of images by the organ of vision ; the 

 way in which the waves of light impinge upon the 

 nervous tissue of the eye, and there leave behind an 

 impression of external objects, to be conveyed to the 

 sensory organ, the brain, comprises a series of vital and 

 physical actions of a marvellously complicated nature. 

 A recent philosophical writer sums up the various 

 operations associated with seeing as follows : " Sight 

 may be defined as an aggregation of colour feelings, 

 and muscle feelings, and the objects of sight groups of 

 such feelings, suggesting other feelings in all indivi- 

 duals. All the sensations which go along with the 

 sensation of sight, interpret it, just as language is 

 interpreted by the brain. That is, a sensation calls up 

 a conception, which is made up of an aggregation of 

 beliefs, and is a link between sensation and action." 

 (Clifford.) In the terser language of a sage, " The eye 

 sees only what it brings with it the power of seeing " 

 (Carlyle); and which, translated, means, that the unaided 

 eye sees but little, and that little imperfectly or incor- 

 rectly unless assisted, as students of the microscope 

 soon discover. The power of seeing, undoubtedly, has a 

 definite limit assigned to it, which differs in most indi- 

 viduals, and varies with increasing age, and from 

 functional causes. The act of seeing is partly volun- 

 tary, and partly muscular, consequently it is capable of 







