128 
DR. T. R. ROBINSON AND MR. T. GRUBB’S DESCRIPTION OE 
use of large reflectors more attainable, cannot but tend to the progress of some of the 
most interesting branches of astronomy. 
Before entering on this description, it may be well to make some remarks on the most 
important recommendations of the Committee, which may be found in the printed 
“ Melbourne Telescope Correspondence.” 
I. The Committee recommend a 4-feet reflector. No doubt a 6-feet would be prefer- 
able ; but it would be five times as expensive, and the execution of it thrice as difficult : 
on the other hand, two of them were familiar with what Lord Bosse’s 3-feet performed, 
and were thus able to say with confidence that an instrument of nearly twice its power 
would be amply sufficient for the work proposed. Two of the Committee wished for a 
5-feet, but some fears were entertained that it might be difficult to mount it on a 
thoroughly effective equatorial, and they decided on the safer course : it has, however, 
turned out that this caution was not necessary, for the actual mounting is strong enough 
to carry a 5-feet, should it ever be required. 
II. They preferred the reflector to the achromatic ; and with good reason. It is not 
probable that an achromatic can ever be made which shall have as much light as a 4-feet 
reflector; and if it could, the cost of it would be tremendous. The late M. Merz, when 
consulted by one of the Committee about a 30-inch achromatic, expressed much doubt 
as to the possibility of making one ; but added that if it were practicable, the cost of the 
object-glass alone would be from £8500 to £9000, and that the equatorial complete 
would not be less than £20,000. What would the equivalent of a 4-feet cost 1 ? Erro- 
neous, I may even say absurd opinions are often expressed as to the relative power of 
these two sorts of telescopes. Thus Fraunhofer says specula reflect “ an exceedingly 
small quantity of light.” Even the elder Struve seems to think that the Dorpat achro- 
matic, 9‘58 inches diameter, “ may rank with the most celebrated of all reflectors, namely, 
Herschel’s.” He cannot mean the “ most celebrated one,” that known as the 40-feet ; 
bat if we even suppose him to speak of the 18-inch front view, the statement is prepos- 
terous*. A speculum reflects 0‘64 of the incident light after being many years in use, 
and even the Newtonian with its double reflection gives 0401, allowing for that inter- 
cepted by the small speculum. And the achromatic does not by any means transmit all 
the light that falls on it. Light is lost in it from two causes : first, from the reflection 
at the four surfaces of the lenses. This can be calculated accurately from Fresnel’s 
formula, which gives that for two surfaces of crown ((t= L521) the transmitted light 
g> 2 =0-9164; for two of flint (^=1-662) g f2 =0 , 8842; and for the four f . g>' 2 =0 , 8122f. 
Hence it is easily inferred that even if its glasses were perfectly transparent, the aperture 
of an achromatic would be to that of its equivalent Newtonian as 1 : 1*42 ; or in other 
words, one equivalent to the 4-feet Newtonian cannot be less than 33 - 73 inches. But 
it must be much more ; for, secondly, all glass absorbs a portion of the light which passes 
* See on this Herschel and South, Quarterly Journal of Science, vol. xx. pp. 286, 293. 
t The loss must he more than this ; for except at the very centre of the object-glass the incidence is a little 
oblique. 
