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ON TELESCOPES AND EQUATORIAL MOUNTINGS. 197 
appears both brighter and whiter than when using the ordinary small spe- 
eulum; the image also appears perfectly free from chromatic dispersion, 
When the quicksilver shall have been replaced by a surface of pure silver, 
the increase of light will of course be equivalent to the proportionably higher 
reflective power of the latter, which, in the absence of good photometric ob- 
servations, may be estimated at the least at a fourth. ‘The same principles I 
propose to apply to the improvement of the Gregorian telescope, with in- 
verted surfaces. 
In the case of the Newtonian reflector, and where the aperture does not 
exceed 12 inches, the prism of total reflexion with plane surfaces, as at pre- 
sent occasionally used, seems hardly to admit of improvement ; but for much 
larger apertures, and especially when we approach the size of Lord Rosse’s 
great telescope, where the requisite size of the prism would involve the pass- 
ing of the rays through about 6 inches of glass, the case is widely different, 
and if the difficulty, not to speak of the expense, of procuring prisms of 
homogeneous and perfectly annealed glass of adequate dimensions did 
not prevent their use, their thickness would go far to neutralize their use- 
fulness. 
_ The arrangement which here first presented itself, as affording some spe- 
cial advantages and permitting of a great reduction in the size of the reflect- 
ing prism, was to construct the prism with a converging power, and place it 
beyond the focus of the large speculum, so that the reflected pencil would 
form a secondary image to be viewed by the eye-glass instead of the primary. 
By adopting an aplanatic construction for the prism, the distinctness would’ 
be preserved, and the entire arrangement better (as having fewer surfaces) 
than the more obvious one of a small plane prism placed a short distance 
_ within the focus, and reaching (so to speak) this image with a long com- 
pound eyepiece of four lenses. Both constructions, however, include two 
obvious disadvantages ; viz. a secondary image, illuminating the surfaces and 
making the field less dark than otherwise; and secondly, and which is of 
more consequence, a very reduced field of view. 
- That form of the reflecting prism which I propose for adoption in the case 
of large Newtonian reflectors, is as follows :—the prism is an aplanatic com- 
pound of negative or diverging power ; this power is of course arbitrary or ad 
libitum, but I prefer that it be such as will about halve the angle of con- 
vergence of the pencil passing through it from the large speculum. As- 
suming this proportion to be adopted, the practical effect will be as follows:— 
the requisite size of the prism will be just halved (linearly), the resulting 
image will be doubled in linear dimensions, and the magnifying power (with 
any given eyepiece) augmented in the same proportion. The length of the 
telescope will indeed be increased, but only by one-fourth of a diameter of 
the large speculum. This arrangement has the obvious advantages of the 
fewest possible surfaces, and no secondary image. It has been objected that 
the field of view is by it lessened. I cannot consider such to be the case ina 
_ practical sense; for even with Lord Rosse’s telescope of 54 feet focus, the 
lowest eyepiece in general use may be doubled in all its proportions, and with 
such lower-power eyepiece and the proposed prism, the magnifying power and 
angular extent of field would correspond with these same as obtainable from 
the combination of the higher eyepiece and ordinary plane mirror or plane 
rism. 
It will be observed that my proposed improvements, so far as described, 
relate only to substitutes for the small mirror of reflecting telescopes ; and 
for so far I consider they may be confidently and advantageously applied. I 
_ see, however, no reason why the same may not be applied to the large specula 
