EDWAED ELSWORTH. 171 



Ether pervades all space not occupied by matter. The 

 disturbance of this ether caused by the aj^pearance of 

 such a body as the sun for instance, propagates a rapid 

 series of undulations or vibrations in every direction at 

 a velocity of 186,000 miles per second. Some of these 

 vibrations are too rapid for the eye, such as the vibrations 

 of heat. Some on the other hand are too slow and can- 

 not be detected by the unaided human eye. 



"Color," says Meldola, "is the visual impression 

 corresponding to oscillation frequency." 



If a series of ether waves were all of one definite rate 

 of oscillation, we would have a pure monochromatic 

 light. 



White light is polychromatic, i. e., it is a mixture of 

 ether waves of different periods and wave lengths. 

 Passing a thin slice of this light through a prism simply 

 sorts out the waves of different length, the shortest 

 apparent waves producing upon the eye the sensation of 

 violet, the longest, red. They increase upward from 

 red to violet. Beyond these, /. e., beyond the red waves 

 we have radiant heat, and beyond the violet nothing 

 visible to the human eye, yet waves which do exert an 

 influence photographically and can be measured. Hence 

 we say that there is a photographic sensitiveness far be- 

 yond the sensibility of the human vision. 



This is illustrated by the photography of the firma- 

 ment, which discloses many stars not discernible to 

 human vision. 



The intensity of these color waves differ. To the 

 human eye the yellow is the most intense ; photographi- 

 cally, the most intense wave is violet. 



So if we looked upon Nature with a photographic eye, 

 violets would appear more brilliant than buttercups. 



Every practical photographer has discovered this. 

 Reds and yellows impress feebly and require longer ex- 

 posure, while violets and blues impress strongly. 



127 



