PHYSIOLOGY OF VISION AND SENSATION OF COLOUR. 2.27 



1. Complete Darkness. 



The colour of the retina when it has been kept in abso- 

 lute darkness is " red " (not purple-red, as I called it in 

 my first paper). I call this colour the ground-colour of the 

 retina or the " vision red." If we observe with the micro- 

 scope (Hartnack's objective 7, without cover-glass) the 

 mosaic of the layer of rods, the great majority of the rods 

 shows the same " red " colour which characterises the whole 

 retina. Between these red ones there appear isolated rods 

 of a very pale greenish tint. If we follow the paling of the 

 retina with the microscope, we see that as the colour of the 

 red rods becomes fainter, they assume a distinct yellowish 

 red, and finally, nearly quite yellow tint. 



2. White Sunlight. 



The retina, after the prolonged operation of the sun's 

 rays, or of bright diffused daylight, appears quite colourless. 

 Under the microscope all the rods appear equally colourless 

 and transparent. 



3. Coloured Light. 



In order to investigate the influence of coloured light on 

 the retina the frogs were kept in glass boxes of various 

 colours, which were exposed as far as possible to daylight and 

 to the sun. At the time at which these experiments were 

 made (December, 1876), grey clouds and dark days were 

 rare, or any long continued sunshine was equally rare. The 

 days were nearly all of about the brightness of white-cloud 

 light, and consequently the following experiments are to be 

 exclusively regarded as made with light of medium intensity. 



1. Red Light. 



The glass used in these experiments (glass coloured with 

 suboxide of copper) absorbs the outermost red rays as far as 

 B, transmits the red and orange from B to D, and completely 

 absorbs the yellow, green, blue, and violet from D to G. 



Macroscopically the ground-colour of the retina appears 

 unchanged. Under the microscope the red rods behave as 

 in the retina kept in the dark, and gradually pale to yellow 

 in quite the same manner; on the other hand, the green rods 

 scattered between the red ones show a much more brilliant 

 colour than the green rods of retina kept in the dark. 



2. Yellow Light. 



The yellow glass absorbs from the outermost red to C, 

 from which point it transmits the red, orange, yellow, and 



