228 
NATURE 
[Fan. 23, 1873 
nn nnn nme EEIEID II ann InPUnEnEInE IEEE REED EERE 
objects, such as the sun and stars, comets, nebulze, planets, 
and so on; we must for this purpose have something at- 
tached to the telescope. Fig. 16 shows a star spectroscope, 
which differs in arrangement only and not in principle from 
other spectroscopes, except in one point to which I have to 
draw attention with regard to this spectroscope. I have in- 
sisted on the importance of the slit ; but you will see in 
a moment that the image of a star, if it is a good image, 
will be a mere point in the telescope, and therefore, while 
Fic. 17.—Direct-vision prism with three prisms. 
a’slit'is not absolutely necessary, it is essential to” have 
some arrangement by which that’ point of light, the 
spectrum of which would be merely a line, and therefore 
not broad enough to enable us to see what the lines are 
which we may expect in the spectra of stars, if they be 
anything like the spectrum of the sun, shall be turned 
intoa band. That has been accomplished by means of a 
cylindrical lens, its function being to leave the light alone 
in one direction, but to turn it into a band in another 
direction, so that when the light of the star gets through 
such a lens, it is no longer a point but a line, and this is 
then grasped by the collimating lens, sent through the 
prisms, and received by the observing telescope, so that 
when you get the image of it in the observing telescope, 
instead of having a line of light so fine that the lines in it 
cannot be distinguished, it is a distinctly broad band in 
which the lines can be observed. As this lens is simply 
a contrivance for enabling the eye to see about where 
there is a line, I submit now, as I submitted some years 
ago, that a proper place for it is close to the eye, 
between the eye and the image, I have been gratified to 
find that, in many of the spectroscopes used on the 
Continent, this arrangement is adopted. 
We have now an idea of the action of the simple prism. 
I will next bring to your notice another kind of prism, 
which differs from the simple one very much as the 
achromatic telescope differs from the non-achromatic one, 
which was the first attempt made at an instrument for 
astronomical observations, Many of you know that the 
object-glass of a telescope, as now constructed, consists 
of two lenses made of different kinds of glass. Of course, 
we have dispersion and deviation at work in both these 
kinds of glass, but the lenses are so arranged, and their 
curves are so chosen, that, as a total result, the deviation 
is kept while the dispersion is eliminated, so that, in the 
telescope, we have a nearly white image of anything which 
Fic. 18.—Direct-yision prism_with five prisms. 
gives us ordinary light, although, as you know, it is by { this direct-vision arrangement is getting into common 
the deviation alone that we are enabled to get the magni- | 
So also in the spectroscope we | 
fied image of that object. 
have an opportunity of varying the deviation and the dis- 
persion. By a converse arrangement we can keep the 
dispersion while we lose the deviation ; in other words, 
we have what is called a direct-vision spectroscope. If 
we take one composed of two prisms of one kind of 
glass which possesses a considerable refractive power, 
and three prisms of another kind which does not re- 
fract so strongly, arranged with their bases the opposite 
way, the deviation caused by the two prisms in the 
one direction will be neutralised by the deviation of 
the three prisms in the opposite direction ; whilst the 
dispersion by the three prisms, exceeds that which is 
caused by the two prisms in the opposite direction, the 
latter dispersion therefore will neutralise a portion only 
of the dispersion due to the three prisms. The final 
result is that there is an outstanding dispersion after the 
deviation has been neutralised, so that when we want | 
to examine the spectrum of an object we no longer have 
to look at it at an angle. No doubt you recollect the 
angle that was made by the light the moment it left the 
prism, but we have an opportunity, by this arrangement, 
of seeing the spectrum of an object by looking straight 
at the source of light: in the application of spectrum 
analysis, especially to the microscope and _ telescope, 
this modification—due to M. Janssen, the well-known 
astronomer, who was the first to bring it into general 
notice—is one of great practical importance, so that in 
any research which does not require excessive dispersion, 
use. I have here another direct-vision arrangement 
which is well worthy of being brought to your notice. 
It does not depend at all upon the principles I have 
just been trying to explain to you. It is called the 
Herschel-Browning direct-vision spectroscope, in which 
the ray is refracted and reflected internally, in the 
prisms themselves. We have therefore, in addition to the 
simple prism which I formerly brought to your attention, 
two other aids to research of extreme value in certain 
classes of observations. The direct-vision spectroscopes 
which are now sold are made on one of the two prin- 
ciples just described ; some of them are made so small 
that they can be easily carried in the waistcoat-pocket, 
and still are so powerful that all the principal, and many 
of the less prominent, lines in the solar spectrum may be 
seen with them. 
Of the special application of the spectroscope to the 
microscope I need say but little now. The spectro- 
scope thus used is a direct-vision one, this form being 
far more convenient for attaching to the microscope. 
The light which illuminated the object in the micro- 
scope was first of all passed through a prism; but in 
later arrangements it passes through the prism in its 
passage from the object. This is obviously a much 
better plan, because, in the first instance, you could only 
deal with transparent objects; but here, as you deal in 
any case withthe light thatcomes from the object itself, 
it is quite immaterial whether the object be opaque or 
transparent, J. NoRMAN LOCKYER 
(To be continued.) 
