THE PHYSIOLOGICAL BASIS OF ILLUMINATION. 



By Louis Bell. 



Presented AprU 10, 1907. Received May 28, 1907. 



The purpose of this paper is to point out that Avith the existing 

 knowledge of physiological optics artificial illumination can be removed 

 from the domain of empiricism and can be made to rest upon constants 

 which have a definite physiological basis and which can be and have 

 been predetermined with reasonable precision. For obvious reasons 

 data which relate to the sensation of sight cannot rank with exact 

 physical measurements, but they can nevertheless be evaluated closely 

 enough to give a reliable basis of judgment in planning illumination 

 to meet any given requirements. 



Except for the aid received from accommodation and in binocular 

 vision from convergence, we see things in virtue of their dift'erences of 

 color and of luminosity. Of these two the latter is by far the more 

 important, particularly in distant vision. Objects of similar luminosity 

 but differing considerably in color blend into the general view in a most 

 astonishing fashion when at any considerable distance. Objects of sim- 

 ilar color but of different luminosity also fuse into the general field, and 

 if color and luminosity are both similar, things disappear in a way that 

 is positively amazing. Small colored areas of moderate luminosity blend 

 even at relatively short range, — a fact which the impressionists have 

 turned to extremely good use, albeit they often transfer to canvas the 

 color vagaries of the tired eye and the effects of simultaneous contrast 

 rather than the fleeting impressions which they hold so precious. One 

 of Monet's landscapes, however, is wonderfully interesting from the 

 standpoint of physiological optics, and especially in the existence of a 

 critical distance, within which the picture loses its magic. 



Practically, therefore, vision depends very largely upon the power of 

 distinguishing differences of luminosity. And since objects in general 

 are luminous only in virtue of light reflected from them, their visibility 

 depends in turn upon their coefficients of reflection. So far at least as 

 problems of artificial illumination are concerned, objects seen do not 



