PROCTOR, ON LIGHT. 167 
object to the measure and number usually adopted ; we shall, 
therefore, accept them for further argument. 
The length of an undulation of violet light is seventeen 
millionths of an inch; the red undulation twenty-six mil- 
hionths, or about one half longer; undulations longer or 
shorter than these not being visible. The colours observed 
in soap-bubbles and other thin films are produced by inter- 
ference of the luminous waves. The colour produced depends 
upon the relation between the thickness of the film and the 
length of a wave of light. A film of air four millionths of 
an inch thick produces the same colour as a film of water 
three millionths, or of glass two and a half millionths of an 
inch thick. Therefore we conclude that the length of the 
light-wave varies with the medium. An undulation in air 
measuring four will measure only two and a half when it 
enters glass, and will again elongate to its former measure on 
its exit. From these premises we may deduce various inter- 
esting conclusions. Faraday found that gold-films were iri- 
descent when they were only one tenth the thickness at which 
air ceases to be iridescent. May we then conclude that 
light, while passing through gold, consists of undulations only 
one tenth the length of those in air? Newton found that 
the thickness of films of a given colour was inversely propor- 
tionate to their indices of refraction. May we then conclude 
that gold has a refracting power in like proportion? If we 
say that luminous undulations, which in air measure twenty- 
two millionths of an inch, look yellow when they enter the 
eye, and in that organ measure one third less, in consequence 
of its refracting power, then we come to the singular conclu- 
sion that the blue sky is yellow, sunshine is red, and the rosy 
tints of evening are not luminous at all till they enter the 
eye. Ifthe colour depends upon the length of the light-wave, 
and the length of the wave depends upon the refracting power 
of the medium through which it is passing, every beam of 
light changes colour; red it may be on its passing through 
the region of the stars, yellow or green it may be when it 
enters the air, blue or vioiet when it enters water, non-lumi- 
nous as it passes through glass. But if light, which we per- 
ceive as violet while it exists in the aqueous humour of the 
eye, was red originally, what colour must that light be which 
we perceive as red? Its undulations in air must be too long 
to be luminous. ‘This introduces us to the solemn thought 
that all this vast universe is dark! Light exists only in the 
eye. It is only a sensation, a perception of that which in 
nature exists as a force capable of producing a sensation. 
We would feel grieved at the thought of light and sound 
VOL, III.—NEW SER, N 
