THE GREAT MELBOURNE TELESCOPE. 
155 
dust or moisture. D is one of the prisms in situ. E is the observing-telescope, l'l inch 
aperture and 4 - 5 inches focus (the collimator is the same). The manner in which it is 
mounted is more fully shown in figs. 40 & 41. The first of these shows in section the 
prism-box C; L is a steel centre firmly fitting the bottom of C; on it turn the arms F 
and G ; F being outside the box and capable of being turned into any position, G turn- 
ing only through a limited arc, equal to the dispersion of the most powerful prism, by a 
slotting aperture in the side of the box, and carrying the telescope E. K is a circular 
plate, serving to retain the arm G in its place, and also to support the prism. The manner 
in which G and E are connected is shown in fig. 41. A portion of G is ratched, in which 
works the endless screw H, having a micrometer-head to measure fractions of a 
revolution. The teeth were cut upon an accurate engine, each being fifteen minutes 
of arc, and the micrometer-head is divided into sixty parts, each of which is therefore 
fifteen seconds. The revolutions are read on the divided arc seen in fig. 39. It will be 
obvious from the firmness with which these parts are put together that very little vari- 
ation of the readings is to be expected. Three prisms are provided, of which two are 
constructed as shown in fig. 39. The central prisms are of Chance’s extra dense flint, to 
which are cemented outer prisms of light crown. The first of these has the angle of the 
flint =100°, those of the crown 28°; its deviation for D=48° 3', and its dispersion 
from B to G'=5° 5'. The angles of the second are 90° and 29° 15', its deviation for 
D = 34° 40', and its dispersion =3° 23'. 
The third prism is an ordinary one of dense flint, its angle being 59° 50', its deviation 
=48° 15', and its dispersion=2° 59'. Each prism is permanently fixed to a disk of 
brass, which by means of steady pins can always be attached to the table K in the 
same position, and retained there by the milled-head screw F, fig. 39. This method of 
mounting permits the prisms once adjusted (to minimum deviation for the line D), to be 
readily placed, even in the dark, at a definite position with respect to the line of colli- 
mation. The compound prisms have nearly the same deviation as the simple one, 
which is very convenient for the observer : they might have been made exactly the same, 
but it was not thought necessary to fulfil this condition, as the difference is easily pro- 
vided for. As F and G (fig. 40) turn on the same centre, a removeable steady pin, with 
as many holes in the former as there are required positions, connecting it with a hole 
in C, will be quite sufficient.] 
As to the performance of this great telescope, it is unnecessary to add anything to the 
report which the Committee has presented to the Royal Society*, and Mr. Lassell’s 
letter to the President. Its light is fully proportioned to its aperture ; and in defini- 
tion on a fine night it is not surpassed by any telescope with Ahich I am acquainted. 
The movements on its axes are effected with a facility and precision which, when one 
considers its enormous weightf, seem almost incredible; and it appears to keep its 
adjustments with admirable steadiness. If, as I trust, it arrives safe at its destination, 
it will fully repay the magnificent liberality and public spirit of those who ordered its 
* Proceedings of the Royal Society, vol. xvi. page 313. t See Appendix, No. III. 
