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Dr. F. W. Edridge-Green. 



per second, or about seventy times the possible transmission. Of course there 

 would be some loss by absorption in the media of the eye, and, again we may 

 not limit the refractory interval as we have done. It might be ten times 

 smaller and yet not fall below the value apparently associated with audition. 

 But even so, we must recognise that in photopic vision the flash of the 

 electron may often come at the wrong moment and fail to evoke vision. 



An important deduction from this consideration is that a highly dilute 

 visual purple may suffice for the requirements of photopic vision. The fact 

 that dark adaptation of the fovea is feeble supports this view. 



The Effect of Red Fatigue on the White Equation. 



By F. W. Edeidge-Green, C.B.E., M.D., F.E.C.S., Special Examiner and 

 Adviser to the Board of Trade on Colour Vision and Eyesight. 



(Communicated by Prof. W. M. Bayliss, F.R.S. Eeceived April 22, 1921.) 



The white equation, that is the amount of pure spectral red, green, and 

 violet, required to match a simple white, is of fundamental theoretical 

 importance. The apparatus used in these experiments is of similar principle 

 to that of Abney, namely, the selection of portions of the spectrum by slits 

 and their re-combination on a white surface by means of a lens, but with 

 several improvements suggested by Captain Fulton. The source of light is a 

 " Pointolite " lamp of 1000 c.p. The light is focussed on the slit of a 

 collimator, from which it emerges in a parallel beam. It is then dispersed 

 by a compound prism of the Amici type. A lens placed close to the prism 

 focusses the spectrum on the slits, the light being reflected by a mirror 

 placed in the path. A second lens is constructed so as to take the whole 

 spectrum, portions of which are isolated by means of slits. The focal length 

 of the lens is arranged so that an image of the last surface of the prism is 

 projected on a screen, the colour being dependent on the portion or portions 

 of the spectrum isolated by the slits. In order to obtain a long light-path, 

 the light is again reflected by a second mirror before reaching the screen. 

 The intensity of a comparison patch of white light is regulated by an 

 adjustable diaphragm placed in the path of light. The apparatus is used in 

 a dark room free from stray light. 



The three lights used in these experiments were a red of A, 6670-A, 6770 A., 

 a green of X. 5144-X 5156 1.., and a violet of X4250-X4267 A. In making 

 the equation, the red and violet slits are kept unaltered, the equation being 

 made by closing or opening the slit, allowing green light to pass. The size 



