166 PROCTOR, ON LIGHT. 
the reflector is a little white-paper label. While covered by 
the shade the reflector appears of a dead-gray colour. It 
reflects abundance of light, but it wants the reflected shadows 
which are essential to the appearance of lustre, and while it 
is thus uniformly illuminated we easily perceive that the 
white paper reflects more light than the silver. 
As the uniform light obtained by the use of these white- 
paper shades very much facilitates our estimate of the reflect- 
ing power of the objects under them, we will do well to com- 
pare the laminated mica and carbonate of magnesia with the 
white paper and silver. Under the other paper shade I have 
placed a small article of polished silver, just to draw your 
attention to the remarkable analogy between the appearance 
of a polished surface with this uniform illumination and the 
dead-white silver under ordinary circumstances. 
Sir D. Brewster, and other writers on optics, give the 
length of a wave of white lght, the number of undulations 
in an inch and the number in a second, calculating it as 
the mean of the number of undulations in the coloured rays, 
apparently forgetting that it is not the mean but the sum of 
the colours which forms white light—the mean being, ac- 
cording to Brewster’s own table, yellow, with a tinge of 
green; various writers have, probably, copied from the same 
source without investing thought upon the subject, one in- 
dication of which is, that several say so many millions of 
millions, whereas it would be more natural to say so many 
billions. I will just give you Brewster’s figures, and then 
pass on: 
No. in a second. 
Length in parts of an inch. No. in an inch. Millions of milliags 
White . .0:0000225 44444. 541 
Yellow .  .0°:0000227 4.4.000 535 
Yellow-green 0:0000219 4.5600 555 
You observe the numbers given for white light are the same 
that would belong to a colour between yellow and yellow 
green. White light, we may conclude, is not a definite un- 
dulation, nor a definite mixture of undulations, but a variety 
of mixtures of undulations, in any of which mixtures the 
average length of an undulation is that given by Brewster and 
others, but the number in an inch or a second is incalculable 
and indefinite. The length of the undulations in a pure un- 
mixed colour is probably definite, and we have no reason to 
