364 Scientific Proceedings, Royal Dublin Society. 
The whole eye-piece also with its rack and pinion turns aside 
and the same motion bringing into the field a low power lens, 
which virtually constitutes the whole telescope a finder with a 
field of 22°. 
The A quick motion is obtained by drawing the A screw out 
of gear by the handle, and turning the telescope on its own axis 
by hand. 
The A slow motion is obtained by the milled head working 
into differential gearing. 
The declination movements are obtained by the button head 
(not visible in the Figure), which is geared by a long rod and 
pinion to a toothed are on the mirror frame. 
Now, this instrument being once got into adjustment, and 
set on, say, the sun, the image will remain in the field for any 
amount of time perfectly stationary. Obviously, the attach- 
ment of spectroscopes to such an instrument is an easy problem. 
It should be remembered that as the cushions into which the 
screws drop when placed in position have certain hollows in them, 
the instrument, if once adjusted, will always come into adjust- 
ment when lowered into position. 
The Spectroscope (See Fig. 11), or at least the primecipal instru- 
ment, for it is intended to have other smaller instruments for 
less delicate operations, is similar to one which I have had 
the honour of exhibiting before the Society, and the first 
instrument of its class was made for Prof. Young of America. 
Its peculiarity consists in having the observing and collimat- 
ing telescopes side by side parallel to one another, and the 
one focussing screw actuates the sliding tubes of both tele- 
scopes, thus affecting automatically what ought to be done in 
every spectroscope, but which owing to the trouble is rarely if 
ever practised—I mean simultaneous focussing. The power of 
this spectroscope is variable from 2 to 4 or 6 or 8 or 10 prisms, 
each of 60° heavy flint glass. The motion is automatic, that is 
movement of the one button head actuates the whole number of 
prisms, and in their proper proportions, 
