THE GREAT MELBOURNE TELESCOPE. 
129 
The law of this transmission is 
J p-nt 
I being the intensity of the emergent light, t the thickness of the medium, and n a con- 
stant depending on the nature of the medium and the colour of the light. In optical 
glass it may be supposed the same for each ray. The form of the equation shows that 
the intensity diminishes very rapidly as the thickness increases ; and since this last is as 
the diameter of the object-glass, we shall soon come to a size which will not have more 
light than a Newtonian of equal aperture. This size could be easily determined if we 
knew n; but I have found no information on the subject except what I have got from 
some measures made by the late Lord Rosse and Mr. Grubb, several years ago, for a 
different purpose. Lord Rqsse’s specimen was a reflecting prism of English crown; 
Mr. Grubb’s were two London plate, one French plate ; and the fourth he described as 
a reflecting prism, but with no note of the quality of its glass. I assume it to have 
been crown, as flint of such size was then rare in England. The intensity of the trans- 
mitted light was measured by Bunsen’s photometer, and the loss by reflection is com- 
puted, assuming p to be that given above for crown. The expression of n is 
log: e 2 — log’ I 
ry^ — ? <2 — . 
t x modulus 
The data are — 
Lord Rosse .... 
. . 1-0-746; 
t= 1-125; 
hence n— 0-1829 
French 
. . 0-805 ; 
0-750; 
0-1728 
London 
. . 0-860; 
0-300; 
0-2142 
London 
0-600; 
0-1446 
Reflecting prism . 
. . 0-810; 
2-000; 
0-0617 
0-1552" 
This last is undoubtedly too small ; and possibly some false light was present in the ex- 
periment. I, however, retain it not to overstate my case. The error in I should not 
exceed ^ of the whole : f is in some degree doubtful, from p not having been specially 
determined for each specimen. All but the last, however, are known to be very near 
what I have assumed, and the error of g 2 is but of that of the assumed p. We will 
take 0T5 for n. As to the t of an achromatic, it will be sufficient in this discussion to 
take its mean thickness. For this Mr. Grubb measured for me the mean thickness of a 
fine 12-inch object-glass of 18 feet focus, which he made some years ago. It is L75 inch, 
whence that of a similar lens of aperture A is — * * ^ 5 , and the expression of its intensity 
i= log-^G-goge^xe-^^^ 0 ®- 1 ^' 3399 ^. 
If in this we put A=33 - 73 inches, we find that I will be only 0-3883, and that such an 
object-glass, instead of being equiluminous with the 4-feet reflector, will be equal to one 
of 3 7 if inches. For an achromatic of 48 inches, if such a one could be made, I would be 
