538 E. BECQUEREL ON THE CONSTITUTION 
second, the results of my late researches on the chemical rays 5 
the third, the results obtained on the phosphorogenic rays; and, 
lastly, in the fourth will be summed up the principal properties 
ot these different rays, and some considerations on their mode — 
of action. 
When I began this work my aim was to comprehend in it 
the study of the calorific rays, in order to complete the question, 
by studying all the radiations which accompany light: not having 
been able to complete this design, I have chosen, in the first 
place, to publish the results which I have obtained respecting 
the other radiations. 
§ I. 
Without entering upon the composition of light which is to 
be found in all works, I will confine myself to the relation of the 
following facts. If we refract a pencil of solar rays through a 
prism of flint-glass, and receive the oblong refracted image on a 
white card, seven kinds of colours may be pretty clearly distin- 
guished, or seven parts of the image, each of which is coloured 
nearly of the same tint; these colours are red, orange, yellow, 
green, blue, indigo and violet; this last being that of the most 
refrangible rays. Figure 1. Plate IX. gives an idea of the luminous 
spectrum. The limit on the side of the red is evident enough, 
whilst on the side of the extreme violet it is more confused ; and 
even beyond the extreme violet, which is at H, we still see a 
slight gray tint, which becomes weaker and weaker. 
If we compare the luminous spectra obtained by means of 
prisms of different substances, we see that the same colours do — 
not occupy proportional spaces. 
The luminous spectrum is not continuous throughout; it is 
traversed by dark lines which separate the coloured bands. This 
phwnomenon appears to have been first observed by Wollaston, 
but it was Fraunhofer who gave an accurate description of it. 
The last-mentioned philosopher having introduced a pencil of 
solar rays into a darkened room through a narrow and vertical 
aperture made in the shutter about one millimetre in width and 
several centimetres long, made these rays fall on a prism of flint- 
glass placed before the object-glass of a theodolite, so that the 
edge of the prism was parallel to the aperture of the shutter, and 
the angle of the incident ray on the prism was equal to that of 
the emergent ray; that is to say, that the prism was placed at 
