62 Grupp—On a New Form of Equatorial Telescope. 
through, but quite disconnected from, the floor of the room. The instrument is now in a condition for 
observing, supported on a solid stone pier, and with its object glass in the open air under most fayour- 
able conditions for observation, while the observer is situated most comfortably, and perfectly sheltered, 
inside the observatory. It should be borne in mind that as there is in this case no open window, or, 
indeed, any opening in the wall (as the frame is so constructed as practically to keep out draughts), the 
telescope is not under the unfayourable condition that it would be if simply placed at an open window, 
where currents of air of differing temperatures just meet and mix. All objects from zenith to south 
horizon, and from E. to W., are now available for observation. 
‘“‘ For setting the instrument there is a right ascension circle divided to 2 m., and reading to 10 sec., 
at the upper end of the tube, and there is a declination are on the mirror frame, read by the microscope 
from the eye-piece end, and by a peculiar arrangement this microscope also serves for illuminating the 
declination arc. 
“The whole eye-piece also, with its rack and pinion, turns aside, the same motion bringing into the 
field a low-power lens, which virtually constitutes the telescope a finder with a field of 2°. 
«The right ascension quick motion is obtained by drawing the right ascension screw out of gear by 
the handle, and turning the telescope on its own axis by hand. 
“The right ascension 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 length of time perfectly stationary. Obviously, the attachment of spectro- 
scopes 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 mstrument, if once 
adjusted, will always come into adjustment when again lowered into position.” 
One of these instruments, of about 4 inches aperture, has been erected at the 
new Crawford Observatory in Cork, and has proved so useful and convenient, not 
only for such special work as originally intended, but also for general observa- 
tions, as to create a general desire that the same principle could be applied to 
instruments of a large size. I did not see my way, however, to do so, as it in- 
volved the figuring of a mirror of at least 50 per cent. greater diameter than that 
of the objective; say, for such an objective as the Vienna (27 inch), 40 inches in 
diameter. Although I do not say that this would be impossible, there is no doubt 
that as no mirror of anything like this size has ever been attempted, success 
would be problematical, and, even if attained, the cost would be enormous. I 
therefore dismissed the idea, so far as very large instruments were concerned ; but 
I did so with great regret, for it was in its application to them that I considered it 
would be most valuable. Afterwards it occurred to me that some advantage would 
be gained by placing the mirror behind the objective. This would render the con- 
struction of the mechanical part a little more complicated, but would reduce the 
necessary size of the mirror, and also reduce the effect on the image of any resi- 
dual error of the mirror. I found, however, I gained very little advantage by this 
change, unless I placed the mirror a considerable distance from the objective, in 
which case the mechanical arrangements became cumbersome. 
Lately, however, it has occurred to me that by using what is generally spoken 
