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arranged for in every one of the spectroscopes used | 
at the present day. And here again we get an 
NATURE 
found out that it was not at all as Newton had represented 
it. Newton told us in fact that the sunlight was con- 
idea of the enormous patience which is necessary tinuous, that is to say, that the spectrum was one in 
in these matters, for we had to wait a century and a 
quarter before the next essential point was hit upon 
in the construction of a spectroscope. Newton made 
a round hole in a shutter for his experiments, but we now 
_know that he ought not to have done that ; he ought to 
have made a slit. But this did not come out until 1802, 
when Dr. Wollaston, by merely using a slit instead of a. 
round hole, made a tremendous step in advance. You 
will see the importance of this in a moment. If we take 
a cylindrical beam of sunlight and put a prism ia the path 
of the beam, we observe that the spectrum is not a 
pure one ; but if we change the round hole for a slit, we | 
which there was no break in the light which flowed out 
to every part of the spectrum, from the extreme red to 
the violet. When Dr. Wollaston tried the slit he found, 
however, that the spectrum, instead of being that rainbow 
_ band of light which you have seen, was really broken by a 
_ succession of fine—beautifully fine—black lines. 
These lines were observed by Dr. Wollaston, but it 
was not till 1814 that we find them mapped out with the 
greatest care, to the number of 576, bya German optician 
named Fraunhofer ; hence they are termed “ Fraunhofer 
| lines,” the principal ones being lettered A, B, C, &c. 
If we say, then, that spectroscopic inquiry dawned with 
obtain a spectrum of the greatest purity ; the red, blue, | Newton, certainly the sun began to rise with Fraunhofer, 
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green, and violet, instead of overlapping and destroying 
the beauty of the spectrum, show distinctly as simple 
colours, each one speaking for itself on the screen. By 
using this narrow slit instead of the round hole which 
Newton made in the shutter, we got the first idea of the 
tremendous importance of spectrum analysis; for no 
sooner had Dr. Wollaston examined the sunlight 
with the new arrangement, as Newton had done a cen- 
tury and a quarter before with the old one, than he 
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for he, no longer content with getting a sunbeam through 
this slit, and finding out and measuring with most ad- 
mirable accuracy these 576 lines in that band of colour, 
turned his telescope to the moon and the planets, and the 
different stars ; and he discovered that, in the case of the 
stars, the positions of the lines varied considerably from 
those they occupied in the spectrum of the sun ; and this 
is one of the most important discoveries which has been 
|made during the present century in these matters. In- 
Sh. Pb 
Fic. 12.—Upper diagram, Spectrum near D : Lower diagram, Spectrum near E. 
deed, it is the foundation of very much of the later more | know, and it is not too early to place this before you, that 
detailed work. these black lines indicate regions in the spectrum where 
The solar spectrum then, as we have said, far from being | there is no light. Ifthe light is perfectly continuous, so 
continuous, is crossed by an almost innumerable number | that every ray of light is enabled to register itself at the 
of dark lines, some being fine and others thicker and | end of the telescope, by painting an image of the slit, you 
blacker. Fig. 12 shows a small portion of the spectrum in | will get a continuous spectrum ; but supposing, for in- 
the yellow and green. Other observers, such as Kirch- | stance, that the whole of the yellow light were absent, it 
hoff, Thalén, and Angstrém, have worked at these dark | is clear that the spectroscope, if it does its duty well, will 
lines, and have drawn most beautiful and elaborate maps, | give you blackness where the yellow light is absent. We 
showing at least 2,000 lines of various thicknesses. | do not find that the whole of any particular colour is 
We have now to pass on from 1812 to the year 1830, | absent, but here and there, scattered over all the colours, 
when Mr. Simms, an optician of world-wide reputation, | there are these places where the rays of light do not come 
made another very important improvement in the spectro- | to tell their story. This is the explanation of the Fraun- 
scope. Instead of merely using a prism and observing | hofer lines in the solar spectrum. In the light which we 
the slit with the naked eye, he placed a lens in front of | get from the sun, certain of the rays which we may sup- 
the prism, so arranged that the slit was in the focus of pose ought to come to us, do not come, and we get no 
the lens. The light which is allowed to pass through the news from them. We do get news of some of the other 
slit is thus turned into a cylindrical beam, and thus rays, which show us the various shades of blue, of green, 
travels through the prism; then, instead of having | and soun; but hereand there a ray, which possibly might 
merely the eye to observe the spectrum, there is another | have come if it were not better employed, does not come, 
lens which grasps the circular beam and compels it to | and therefore the image of the slit cannot be painted. 
throw an image of the slit, which may be magnified at 
pleasure. The very great importance of this construction 
is at once obvious, if you think for one moment of the 
figure showing the linesin the solar spectrum, We now 
I am glad to say that we know a little more about 
these lines than we did some years ago. You may 
imagine the enormous mystery—the wonderful reverence 
almost—with which this question of the Fraunhofer lines 
