M.B. 88 



In fact the mechanism by which light is transformed into 

 nervous action has not been satisfactorily explained. 



Unlike the photographic plate the retina is not uniformly 

 sensitive. Omitting the blind spot (the entrance of the optic 

 nerve) the sensitiveness of the retina diminishes as the yellow 

 spot or macula is departed from ; but even this is not constant, 

 it varies for light, form and colour. A white object is seen at 

 a greater distance from the centre than a blue, a blue than a 

 red, and a red than a green, and these distances again vary with 

 the amount of illumination. They can be recorded by an 

 instrument called a perimeter on diagrams showing concentric 

 circles, which correspond to parallels of latitude on a globe, with 

 radiating lines for longitude, but it must be remembered these 

 circles and lines are in the concavity of the globe not on its 

 convexity — " on the inner surface of a hemisphere whose pole 

 is the point of fixation." The mapped out area is the field of 

 vision for the particular light, form or colour. 



Vision. 

 This includes — 



1. The form sense — visual acuity or the power of dis- 



tinguishing objects. This is roughly and somewhat 

 arbitrarily estimated as the ability to define sharply 

 under good illumination objects at from about ten 

 (10) inches to infinite distance from the eye, seen 

 under an angle of five (5) minutes. 



2. The light sense — or the faculty of recognising different 



luminous intensities. 



3. The colour sense — or the appreciation of the smallest 



differences of perceptible contrast of colours or of 

 tones. 



The Colour Sense. 



Theories of the mechanism of colour vision are chiefly based 

 on the theories of light, and attempts are made to blend the 

 physical and the physiological — colour mixtures and colour 

 sensations. 



The "Young-Helmholtz" and the "Maxwell" are trichromic 

 theories and assume three sets of nerve filaments in the retina • 

 these filaments correspond to and are excited by the three 

 spectral primaries — red, green and violet, or blue according to 

 Maxwell — but each colour besides acting upon its own particular 

 nerves influences to a certain extent the others. If the three 



c 



