32 REPORT — 1881. 



that the velocity of light is greater ia air than in water — thus completing 

 the evidence in favonr of the undnlatory theory of light. 



The idea is now gaining ground, that, as maintained by 

 Clerk-Maxwell, light itself is an electro-magnetic disturbance, the 

 luminiferous ether being the vehicle of both light and electricity. 



Wiiusch, as long ago as 1792, had clearly shown that the three 

 primary colors were red, green, and violet ; but his results attracted little 

 notice, and the general view used to be that there were seven prin- 

 cipal colors — red, orange, yellow, green, blue, indigo, and violet ; four 

 of which — namely orange, green, indigo, and violet — were considered 

 to arise from mixtures of the other three. Red, yellow, and blue were 

 therefore called the primary colors, and it was supposed that in order 

 to produce white light these three colors must always be present. 



Helmholtz, however, again showed, in 1852, that a color to our 

 unaided eyes identical with white, was produced by combining yellow 

 with indigo. At that time yellow was considered to be a simple color, 

 and this, therefore, was regarded as an exception to the general rule, 

 that a combination of three simple colors is required to produce white. 

 Again, it was, and indeed still is, the general impression that a com- 

 bination of blue and yellow makes green. This, however, is entirely 

 a mistake. Of course we all know that yellow paint and blue paint 

 make green paint ; but this results from absorption of light by the semi- 

 transparent solid particles of the pigments, and is not a mere mixture of 

 the colors proceeding unaltei'ed from the yellow and the blue particles : 

 moreover, as can easily be shown by two sheets of colored paper and a 

 piece of window glass, blue and yellow light, when combined, do not give 

 a trace of green, but if pure would produce the effect of white. Green, 

 therefore, is after all not produced by a mixture of blue and yellow. 

 On the other hand, Clerk-Maxwell proved in 1860 that yellow could 

 be produced by a mixture of red and green, which put an end to 

 the pi'etension of yellow to be considered a primary element of color. 

 From these and other considerations it would seem, therefore, that the 

 three primary colors— if such an expression be retained — are red, green, 

 and violet. 



The existence of rays beyond the violet, though almost invisible to 

 our eyes, had long been demonstrated by their chemical action. Stokes, 

 however, showed In 1852 that their existence might be proved in 

 another manner, for that there are certain substances which, when 

 excited by them, emit light visible to our eyes. To this phenomenon 

 he gave the name of fluorescence. At the other end of the spectrum 

 Abney has recently succeeded in photographing a large number of lines 

 in the infra-red portion, the existence of which was first proved by Sir 

 William Herschel. 



From the rarity, and in many cases the entire absence, of refei'ence 

 to blue, in ancient literature, Geiger — adopting and extending a suggestion 

 first thrown out by Mr. Gladstone — has maintained that, even as recently 



