226 
NATURE 
Pa i = et >. uti) had ts 
JA = 
<< : 2 : “ 
[Xan. 23, 1873 
1 
ON THE SPECTROSCOPE AND ITS 
APPLICATIONS 
III. 
S° far, I have spoken of spectroscopes as spectroscopes 
—as one of the instruments the improvement of 
which should be cared for by every student in science. 
Their applications will come after. As may be imagined, 
spectroscopes are now constructed with one, two, three, 
four, or more prisms, the number depending on the pur- 
pose for which they are to be employed. An _ instru- 
ment with one prism is usually called a chemical spec- 
troscope, for an instrument of this kind is now almost as 
important and essential in a chemical laboratory as a 
balance. Spectroscopes are also constructed with two 
prisms, as shown in Fig. 13; these are used in cases when 
rather more dispersion is desired than can be obtained 
with the one-prism instrument. When, however, any 
accurate and elaborate work has to be done, such as in 
carrying out original investigations, more prisms have to 
Fic.%13.—Spectroscope with two prisms, 
beemployed. The engraving given in Fig. 14 is of an | rate and accurate maps of the solar spectrum ; itis fur- 
instrument which historically is extremely interesting, as 
being the one with which Kirchhoff made his most elabo- 
nished with a battery of four large prisms, which give an 
enormous deviation and dispersion. There is no reason 
Fic, 14.—Steinheil’s form of four-prism spectroscope ; arrangement of slit shown separately. 
why spectroscopes of many more prisms should not be 
employed, except that they require to be worked only 
with strong lights, as light is here so much dispersed or 
spread out that a feeble spectrum would be almost lost. 
As the principle of construction is almost the same in all 
kinds of spectroscopes, we had better commence by a 
description of the simplest form, namely, that with one 
prism, as shown in Fig. 15. It will be seen to consist of 
a circular table, supported by a pillar and three legs, 
carrying three lateral tubes ; the right-hand tube is called 
the collimator, and holds at its outer extremity the fine 
slit, the width of which can be regulated to a nicety by a 
micrometer screw ; the other end of the collimator is fur- 
nished with a lens, which serves to collect the rays of 
light coming from the slit, and to render them parallel 
before falling on the prism in the centre of the table, 
The prism is so placed and fixed by a clamp that the 
light entering the slit from the source of light, shown in 
the figure as a gas lamp, strikes it and leaves it at what 
is called the angle of minimum deviation, a term which 
has already been: explained ; after passing through thé 
prism, in which the light undergoes both deviation and dis- 
uo Bae 
;) ot ee 
* 
