i rebar connected with chemical circulation, he em- 
ployed the spectroscope with great success. Many of 
_ you, at the first blush, would be inclined to say it was not 
very likely that the spectroscope would help us here. If 
it.were a question, for instance, of our own chemical cir- 
culation, we would not relish the idea of being converted 
into an incandescen vapour for the pleasure of testing a 
theory. But, fortunately, there are such things as guinea- 
igs, and Dr. Bence ; 
; Re. by studying 
the vapours of the 
ashes of these ani- 
mals, has arrived at 
some results of ex- 
treme importance. 
His modus operandi 
was as follows :— 
_ He gave the guinea 
pigs chloride of li- 
_thium, and then the 
question was to 
find, by burning the 
ashes of the diffe- 
rent parts of the 
guinea-pigs, —_vari- 
ously removed from 
the fountain of cir- 
culation and from 
the ordinary ducts 
_ of the body, to as- 
certain how long it 
took lithium to get 
absorbed into every- 
part of the body. 
The most distant 
part, as far as cir- 
culation goes, is the 
lens of the eye. If, 
then, we give a 
guinea-pig chloride 
of lithium, then kill 
the guinea-pig, and 
examine the ash of 
the eye lens, say 
three hours after 
the lithium has been 
taken into the sys- 
tem, and if we find 
the lithium line in 
the spectrum of the 
ash vapour where 
no lithium was be- 
fore, that is to say, 
if by means of 
the = spectroscope & 
we see that line 
which we have seen 
characterises the li- 
_ thium spectrum, we 
now that the che- 
Mmical circulation of 
the body is such 
as to take lithium 
_ through the body 
to that particular 
point of the circulation in that time. In the human sub- 
ject Dr. Bence Jones has hit upon a very practical method 
of arriving at something like the same conclusion, by ex- 
amining the spectra of the ashes of cataracts. 
__ So far as I have dealt with the applications of the spec- 
troscope, up to the present time, I have dealt in the main 
_ With the application to chemistry and to physics, in cther 
words, to the examination of light given out by terrestrial 
347 
substances ; but I must now, with your permission, take 
you to the skies, reminding you that at present, I am 
merely dealing with the giving out of light, and with 
light emitted by celestial objects. We shall afterwards 
have to deal with the stopping or absorption of light, by 
vapours and other transparent media when the light 
passes through them. 
I have already referred to the special fittings that were 
necessary for the 
application of the 
spectroscope to the 
Fic. 31.—Sun Spectroscope. 4, Collimator; ¢, Observing telescope ; 4 and M, Two micrometers ; 
I, 25 35 4) 516, 7, Prisms. 
Fic, 32.— Sun Spectroscope arranged fer photography. 
telescope, and I 
think on carefully 
looking at the en- 
graving (Figs. 27 & 
28) representing a 
star spectroscope, 
you will see exactly 
how the spectro- 
scope is applied to 
a telescope. We 
must now go alittle 
more into details. 
One class of spec- 
troscopes, as ap- 
plied to telescopes, 
is arranged for ob- 
serving the spectra 
of the stars, nebule, 
&c., and another 
with a much greater 
dispersive power for 
observing the spec- 
trum of the sun. 
In both spectro- 
scopes the arrange- 
ments employed are 
similar, and resem- 
ble those of the 
instruments that 
have been already 
described. A finder 
on the side of the 
large telescope en- 
ables the image of 
the star to be 
brought cn the slit, 
while, in the case of 
the sun, its image 
is received on the 
slit screen, and any 
part of the image 
may be brought on 
the slit by mere in- 
Zoe 
| 
- om maim || \) ] 
ni | 
spection. 
The spectroscope 
is attached to 
the eye-piece end 
of the instrument, 
ard the image for- 
ed by the tele- 
scope is received on 
the slit plate. Ar- 
rangements are 
necessary in the 
case of the star spectroscope for widening out the spec- 
trum ; this is done by a cylindrical lens (as before ex- 
plained); and for obtaining a spectrum of comparison, 
this is done by reflecting into the instrument the light 
emitted by an electric spark. 
In the star spectroscopes, the number of prisms, and 
the consequent deviation and dispersion, is smal], The 
accompanying woodcuts will make their detailed con- 
struction quite clear. In the case of sun spectroscopes, 
