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DR. T. R. ROBINSON AND MR. T. GRUBB’S DESCRIPTION OF 
only 0‘2842, and therefore even this would not equal the reflector. These conclusions 
may seem startling to many ; but they can only be avoided by supposing an error in n very 
much greater than can be admitted. It is, however, my purpose, in cooperation with 
Mr. Grubb, to institute a series of experiments on the subject which will set the question 
at rest*. 
If we seek any comparison of the light of the two kinds of telescope based on actual 
measures, little is to be found but vague conjectures. Thus : — 1st. M. Otto Struve thinks 
that Mr. Lassell’s 2-feet Newtonian “is nearly equal in light to the Pulkova 16-inch 
object-glass:” this seems to refer to stars only. 2nd. I have examined faint objects 
with Mr. Cooper’s achromatic of 13'6, and with Sir J. Herschel’s front view of 18, and 
have no doubt of the superiority of the latter: the ratio here is T32. 3rd. 72 circular 
inches of the central part of the Armagh Cassegrain showed the fifth star of Orion’s tra- 
pezium as well as a good Cauchoix object-glass of 6 - 5 inch ; this gives T31 for the ratio. 
The only two definite measures which I know are ; 4th. Mr. Potter found that a 4-inch 
object-glass by Dollond transmitted only 0’66 of the incident lightf. Lastly, Amici 
ascertained that an object-glass of 2'5 aperture was as bright as a Newtonian of -f the 
aperture ; this would give 36 inches for the equivalent of the 4-feet reflector ; but what 
has been already stated will show that it would be far short of this J. 
As to defining power, it is certain that specula can be finished to possess this in the 
highest degree, while their great angle of aperture with the absence of the secondary 
spectrum and of diffraction-rings adds greatly to their distinctness. The champions of 
the achromatic rely entirely on its power of showing small stars, which they assert are 
not visible in the reflector. This, however, is not proved ; for it is no proof that because 
a star does not appear in the drawing of a nebula it was therefore invisible, the fact 
being that the observer’s attention was directed to a different object. The effect of this 
is shown, among other examples, by the fifth star of the trapezium, and the dark ring 
of Saturn, which were long overlooked in telescopes which showed them easily when 
they were looked for. Besides, minute points of intrinsic brightness are brought out 
by high powers, but are quite invisible with the low ones which are used for nebulae ; 
but the case is very different with faint objects of sensible diameter. That any com- 
parison may tell the truth, it should be made with the same power, at the same time 
(for many small stars are believed to be variable), and by the same eye. The only real 
advantage of the achromatic is that, from its smaller aperture, it is less disturbed by the 
air’s unsteadiness. But long before it reached a size equivalent to the 4-feet this differ- 
ence would disappear, and a part of it is removed by the arrangement described in the 
next section. 
* The results of these are given in an Appendix. 
t The focal length is not mentioned : I found that the old object-glass of the Armagh circle, 3-8 and 6-2-inch 
focus, transmitted only 0-60. It is a very good one, but the crown is rather strongly greenish. Many of the 
glasses of the elder. Dolloxd have this tint. 
X. A 3-feet object-glass would have 54 feet focal length and would weigh about 5 cwt. As it could be sup- 
ported only at the circumference, it would be difficult to prevent flexure in it. 
