ON INSTRUMENTS AND THEIR USE. 23 



these globules often gravitate to the centre of the 

 cornea, and even after prolonged use of the inclined 

 tube an observer may often be perplexed by the 

 layer of mucus, or a lachrymal discharge covering 

 the surface of the cornea. 



Colour is a special sensation excited in the retina 

 by rays of a definite wave length, and the reason 

 why certain objects are presented to our view with 

 colour is that when white light falls upon a given 

 surface, some is absorbed, the remainder being 

 reflected. If the green rays are reflected, then the 

 object appears green, and if the red rays are alone 

 reflected, then the object will be red. 



The generally accepted theory of colour percep- 

 tion is based on the assumption that three kinds of 

 nerve fibres exist in the retina, the excitation of 

 which produces sensations of red, green and violet, 

 and that modifications of these three sensations 

 yield all intermediate tints. 



This theory will explain some of the phenomena 

 of colour blindness ; if the nerve fibres which give 

 their special sensation are paralysed, or are absent, 

 the sensation only of the complementary tint will 

 be transmitted with all the defects of the eye. It 

 must not be forgotten that many phenomena con- 

 sist more in errors of judgment than in absolute 

 error of form or sensation. 



Now in regard to errors of judgment, we must 

 admit that all our estimations are made by compari- 

 son. In magnitude we are guided by the size of 

 the retinal image as determined by the visual angle 

 for position we must have some starting point; 

 and as for distance, every one knows how delusive 

 an inexperienced estimate of this is. At sea, a 



