Se nnn 
QN THE SPECTROSCOPE AND ITS 
APPLICATIONS 
Il. 
WE now arproach Newton’s great discovery, which is 
this :-—“ Zhe light of the sun consists of rays aiffe- 
rently refrangible ;” that is to say, if we take a beam of 
sunlight, and make it pass through a prism, we shall get 
colours of different refrangibility. We see then that 
if, instead of two coloured beams, we pass one of per- 
fectly white light through the prism, the action of the 
prism is at once to turn that beam into a beautifully 
coloured band, which will remind you of a rainbow. It 
was this which Newton didin a dark room, which led him 
to his important discovery. White light is compounded 
of light of different degrees of refrangibility. But how 
is it possible to show the truth of Newton’s assertion that 
white light is compounded of these different colours ? We 
can do so by simply placing in the path of the coloured 
beam which you see passing through the room, another 
prism placed in a contrary direction, as shown in Fig. 10; 
you see in a moment that we get back white light ; for the 
second prism exactly neutralises the effect caused by the 
first, and the ray proceeds as if nothing had happened. 
Possibly you may ask, is it true that white light is built 
up of all colours? That question can be answered to a 
certain extent by an experiment of a different order, If 
Fic. ro.—Recomposition of white light by means of a second prism. 
a disc, divided into sections and coloured with the 
principal colours of the spectrum as shown in Fig. 11 be 
taken, and if it be true that the idea of white light is 
simply an idea built up by the eye, because we have all 
these multitudes of light waves perpetually pouring into 
it with a velocity that is very much greater than anything 
which can be translated into words, surely we should get 
something like this effect also if we were able, by rapidly 
rotating this screen, to obtain a more or less perfect 
substitute for white light. The coloured disc being made 
to rotate rapidly, you see we obtain something like an 
approximation to white light, though the white colour 
does not come out so clearly as it might do. Now I 
NATURE 
am very anxious that you should see that this is really | 
an effect due to the flowing in of light from different 
parts of that wheel into the eye, and so forming this 
compound impression, which is conveyed to the brain ; 
and so if instead of illuminating the disc continuously 
by the electric lamp, or by sunlight, it is illuminated 
intermittently, by an electric spark, you would see 
that although the disc is rotating rapidly all the time, | subject which we owe to Newton are, first, the explination 
each separate colour is now discernible, and the disc 
appears to stand still. The reason of this difference is, 
that in one case the rotation of the wheel builds upa 
compound image in the eye, and in the other case it 
cannot do so, because the flash of the light is much more 
rapid and instantaneous than the rotation of the wheel. 
There is one more experiment which can be easily 
made, to show that all the beautiful colour which we get 
in nature is really reflected after all, and that if our sun- 
light, instead of being polychromatic—that is to say, com- 
pounded of all these beautiful colours—were monochro- 
matic, or of one colour only, the whole expanse of creation 
would put on a very different appearance from what it 
does. If, instead of illuminating “ diagram, the letters 
of which are of different bright colours, by the white light 
of the electric Jamp, we illuminate it by a light that only 
contains one colour—by the yellow light of sodium, for 
instance, and then look at the diagram, you will see that 
seme of the letters upon it are almost invisible, whilst 
others are very clear, the yellow light only allowing a dif- 
ference to be seen of more or less depth of shade, there 
being no difference in colour. But when we allow the poly- 
chromatic light from the lamp, or as we get it from the sun, 
to shine upon the diagram, you at once see that all these 
letters are of different colours, and burst out, as it were, 
into beauty, This experiment feebly indicates the advan- 
tage we possess in living in a universe lit by white or 
polychromatic light, instead of light which is merely blue, 
or yellow, or any other single colour. 
Hitherto we have spoken only of refraction. I now in- 
troduce the word dispersion, which represents simply a 
measure of different refractions, or the difference between 
the bending of the red and the violet rays of light. In an 
ordinary spectrum the difference between the red and the 
Fic. r1.—Recomposition of white light by means of a rapidly revolving 
[disc coloured in sections. 
violet is the difference of the refraction of those two 
colours by the prism, and the angle which the red, or 
yellow, or other colour forms with the original path of the 
compound-beam is called the angle of deviation. 
There is one other consideration which we owe to 
Newton. In his very first experiments, that great philoso- 
pher discovered that the quality of the spectrum depended 
very much on the following consideration :—If I wish to 
get the best possible effect out of a prism and the purest 
possible spectrum, I have so to arrange it that the particular 
ray which I wish to observe, whether the yellow, the blue, 
the green, or any other, leaves that prism at exactly the 
same angle as the incident compound ray falls on it. 
This angle is termed the azgle of minimum deviation, 
The two things, therefore, of greatest importance in this 
of the dispersive power of the prism; and next, the 
pointing out the extreme importance of arranging the 
prism, so that if we want to observe any particular part 
of the spectrum, the rays constituting that part of the 
spectrum should leave the prism at the same angle as 
the white light falls on it. 
It is very curious, however, that Newton, although he 
made many experiments on prisms, really omitted one of 
the most important points, which you will see carefully 
