Sir David Brewster on the Lines of the Solar Spectrum. 385 
for it is quite possible that a body may transmit light perfectly 
white, and yet exercise a definite action in absorbing various 
parts of the spectrum. The only physical condition which 
is necessary in this case is, that the sum of all the rays thus 
absorbed, shall constitute white light. 
The first substances which I examined were sulphur and 
iodine vapour. The sulphur attacked the violet end of the 
spectrum with great force, and, when combined with arsenic, 
in the form of native orpiment, its absorptive power for the 
same colours was greatly increased. Even with the thinnest 
film that I could detach, and not exceeding the two-hundredth 
part of an inch, the spectrum was, as it were, cut sharply in 
two near the boundary of the green and indigo spaces, and 
this body possessed the very uncommon property of having 
nearly the same colour at small as at great thicknesses. By 
increasing the thickness, the absorption advances almost im- 
perceptibly from the remaining blue border, and if the trans- 
parency continued, the transmitted light would certainly be- 
come red at great thicknesses,—a property which may be 
communicated transiently to the thinnest plates, merely by an 
increase of temperature. 
The iodine vapour acted powerfully upon the middle of the 
spectrum, and, by an increase of thickness, gradually extended 
its absorption towards both extremities; but more rapidly to- 
wards the violet one, so as to show that the final colour must 
be a homogeneous red*. 
In so far as these two experiments went, they were highly 
favourable to the speculation which had at first presented it- 
selfto me. My attention was now directed to the action of 
gaseous bodies, and the first trial which I made was with 
nitrous acid gast. The result of this experiment completely 
destroyed the hypothesis which had appeared so plausible, 
and presented me with a phenomenon so extraordinary in its 
aspect,—bearing so strongly on the rival theories of light,— 
extending so widely the resources of the practical optician, 
and lying so close to the root of atomical science, that I am 
persuaded it will open up a field of research, which will ex- 
haust the labours of philosophers for centuries to come. 
The spectrum of Newton, and of all the philosophers of 
the 18th century, was a parallelogram of light, with circular 
ends, in which the seven colours gradually shaded into each 
other without any interruption. The illumination was a maxi- 
mum in the yellow rays, and the light decayed by insensible 
* [See Lond. and Edinb. Phil. Mag., vol. ii. p. 362. ] 
+ [See Lond. and Edinb. Phil. Mag., vol. ii. p. 381 
By 
Third Series. Vol. 8. No. 48. May 1836. 2 hh 
