THE GREAT MELBOURNE TELESCOPE. 
153 
illuminating lamp ; and inside the lantern, but without touching its bars, stands the 
concave illuminating reflector K, supported by two studs, L, which are attached to the 
ring M. This is on its upper edge cut into contrate teeth, which are acted on by the 
pinion and milled head O. 
The nozzle of the lamp being made to turn in the conical tube I, and this latter being 
(by means of the tube H) revolvable on the lantern part, it follows that the lamp can be 
set vertical in any position of the telescope, while the reflector K can be revolved by 
the milled head O till it throws the lamp’s light on the micrometer-wires. Thus one lamp 
is, by very simple management, in every case sufficient. In the Cassegrain telescope there 
is an eyehole so adjusted that it excludes all light except what comes from the object- 
speculum : now if the central opening of K be no larger than what will pass that light, 
it is evident the field will be dark, while the rays of the lamp intersecting in the focus of 
K will illuminate any object placed there, and of course the wires in whose plane that 
focus is. The same can be arranged, though not so easily, for a Newtonian. N in 
fig. 32 is a large milled head, by turning which, Q, the box of the micrometer is made 
to revolve. It works a pinion which acts on the wheel R, fig. 33. S in both figures is 
the position-circle reading to a minute by the verniers T, T\ The optical part is peculiar, 
and such as had been long since applied by Dr. Robinson to the micrometer of the 
Armagh Cassegrain to obtain a larger field than was given by the usual positive eye- 
pieces. A field glass P, common to all the eyepieces, is placed at a proper distance 
before the wires, and the eyepieces are single lenses placed behind them. This greatly 
enlarges the field of view without producing any sensible distortion of the image in a 
Cassegrain*. The micrometer is provided with powers from 300 to 600.] 
In using it the small speculum must be fixed in the position which brings the third 
image into the plane of the wires, and focusing must be effected by sliding the eye-lens. 
What we have described comprises all which was included in the contract, but it 
seemed to the Committee that the instrument would be incomplete without a photo- 
graphic apparatus and a spectroscope, and they ventured to add these on their own re- 
sponsibility, sanctioned by the opinion of the President and Council. Some photograms 
of the moon and stars taken with a temporary apparatus, were considered by the highest 
authority in this matter, Mr. Warren De La Rue, to be of such good promise that they 
directed a very complete one to be provided similar to the one used by that astronomer, 
with only such modifications as the great bulk of the telescope makes necessary. 
[The most important details are shown in figures 35 to 38, of which the first two are 
on a scale of 1 inch to the foot. Fig. 35 is a plan, and fig. 36 half section, half eleva- 
vation of it. AAA is the angle-iron forming the upper end of the telescope, to which 
is attached the steel tripod frame B B B, by the thumb-screws x, x, x. This tripod 
supports the pair of brass tubes e,f; the steel arms being connected with the outer tube 
* In the Armagh instrument the values of a revolution of the screws, deduced from ten, twenty, and thirty 
revolutions of them by transits of circumpolar stars, differ from the mean, — 0 ,,, 028; +0' ,- 054; and — 0 ,,- 026, 
which are quite unimportant. — E. 
X 2 
