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NATURE 
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125 
ON THE SPECTROSCOPE AND ITS 
APPLICATIONS * 
| eke field of research which has been opened up by 
the spectroscope is one with which we have so 
recently become familiar, that it may almost be said 
that twenty years ago, a course of lectures on the spectro- 
scope would have been animpossibility. The instrument, 
as we now know it, was only then inembryo, and even at 
the present time, although immense strides are every day 
Fic. 1.-—Geometricaliform of the prism. 
being made, the science of spectroscopy must still be 
considered in its infancy. And yet, so far as one can see 
now—it is always very easy to prophesy after the event— 
there seems very little reason why lectures on the spectro- 
scope should not have been given two centuries ago ; for 
Fic. 2.—Prism mounted on a stand. 
nearly two centuries have elapsed since the immortal 
Newton made his classical researches on the action of 
aprism upon sunlight. You may, perhaps, be inclined to 
ask, how it could take 200 years for the knowledge of the 
prism, and of the wonders that can be worked by it, to 
become part and parcel of our common stock of infor- 
mation? If you ask me to explain this, I tell you 
candidly that I cannot ; but there is this grain of comfort 
connected with it which none of us should forget: we 
* Revised rom the series of Cantor Lectures, delivered in 1869. 
may almost say for certain that Newton and his suc- 
cessors would have brought a great deal more out of the 
prism than they did, if they had given a little more 
attention to it, and had tortured it as they did other 
things ; that those who follow us will point to us and say 
the same ; they possibly will say that in the roth century, 
men of science, in working and experimenting, saw a 
great many things, and chronicled them, but did not care 
to go any further with them. This is very true; and the 
result is, that work is not done which might be done if 
we were more receptive and original in our methods of 
investigation ; that is to say, if we trusted Nature more 
and ourselves less. 
Fic. 3.—Refraction‘ot light. 
I propose that the first part of this lecture should in 
the main consist of an account of the prism and the 
principles of the spectroscope, and then of a description 
of the various kinds of spectroscopes which are now 
employed. I hope afterwards to go somewhat in detail 
into the applications of the spectroscope, not only with 
regard to terrestrial matters, but also with regard to those 
problems which we may possibly consider much grander, 
problems dealing with those celestial bodies which are 
sufficiently our neighbours to send us light. 
Obviously, the first question we have to answer is this, 
What is a spectroscope? This I answer by saying that 
Fic. 4.—Explanation of the bent stick. 
a spectroscope is an instrument in which the action of a 
prism or a combination of prisms is best studied. The 
next question, then, that arises, is, What is a prism? 
The accompanying figures (Figs. 1 and 2) will give a 
good idea of what is meant by a prism, and little time 
need be spent in description. It is usually a piece of 
glass—though it need not necessarily be so—bounded by 
five surfaces, two of which are parallel to each other— 
though they are not necessarily so—and three of which, 
bounded by parallel edges, cut each other at different 
angles ; it is in reality shaped like a wedge. The impor- 
tance of these different angles you will see by-and-by. 
