1864.] Srewarr on Radiant Light and Heat. 591 
such an arrangement. Now, let a ray of light, as in Fig. 2, strike 
obliquely against the side of the prism, enter it, and pass through. 
It will be greatly deflected by this process, so that its line of exit 
will differ very much in direction from that of incidence. This is 
sufficiently well shown in our figure, but there is yet something more. 
All rays are bent, but rays of one colour are deflected differently 
from those of another. If, now, the ray which impinges upon the 
prism be a single one of white light, that which leaves it will be no 
longer a single ray, but rather a pencil of rays, in which we shall 
obtain, in a separated condition, all those colours which together 
constitute white, because each one has been bent in a different 
direction. 
We may now easily comprehend what is meant in optics by the 
term ‘“‘ Spectrum.” In order to do so let us recall before us the ordinary 
photographic camera, not however to be used in obtaining the likeness 
of a landscape or of a friend’s face, but only that of a slit illuminated 
by white light. If our arrangement be the ordinary one, we shall of 
course obtain as an image on the screen which is placed in the focus 
of the camera a single line of light; but if we interpose a glass prism 
between the illuminated slit and its image, each individual colour 
which goes to form white light will be bent in a different direction by 
this prism, and will give rise to an image that will be thrown upon a 
different part of the screen. Instead therefore of having one image of 
the slit of light upon the screen, we shall in reality obtain a number, 
each having its appropriate colour. The image will, in truth, form not 
a line at all, but rather an oblong space differently coloured at each 
part. This oblong illuminated coloured space is called a “ spectrum ;” 
and if the line of light whose image we are viewing be that which 
proceeds from a slit illuminated by the sun, then we shall obtain the 
solar spectrum. 
Let us arrange so that those rays which are least bent may le to 
the left, and those most bent to the right, and we shall then have 
colours proceeding in the following order from left to right: viz. red, 
orange, yellow, green, blue, indigo, violet. Red therefore is the least, 
and violet the most, refrangible ray ; but at the same time the wave- 
length is greatest towards the left, that of red being about s7d5s, 
while that of violet is only szio0 of an inch. We thus see that light- 
waves are extremely small as compared with those of sound, and 
also that there is hardly one octave compreended in the visible 
spectrum, since the wave-length of violet is rather more than half that 
of red. 
But while this embraces the whole of the visible solar spectrum, 
we yet know by means of the thermometer that there is a very con- 
siderable heating effect to the left of the red, thus denoting the presence 
of invisible rays; and we likewise know, through certain experiments 
which we cannot here detail, that the spectrum extends very much to 
the right of the violet, the invisible region in this direction being 
chemically powerful although its heating effect is but small. We 
now come to a very curious fact, first pointed out by our countryman 
Wollaston and afterwards by Fraunhofer. It is found that the solar 
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