12 
MR. BARLOW ON THE PERFORMANCE 
and the light is of course also greater ; still, however, when I consider that I 
have been comparing with two telescopes, one of twenty inches aperture, and 
the other of twelve inches, and each of them twenty feet in focal length, or 
nearly, while my telescope is barely eight inches aperture, and only twelve 
feet in length, I cannot but consider the comparison as highly satisfactory. 
In addition to the above observations, which have been certainly highly gra- 
tifying to myself, I have also had the honour of showing the instrument to 
many persons, both Englishmen and foreigners well acquainted with astro- 
nomy, and in every instance the practicability of the principle of construction 
has been admitted ; a point by no means generally granted when the sug- 
gestion was first advanced. 
Other obstacles also, independent of the arrangement of the lenses, were 
foreseen, which time is gradually dissipating ; such as the difficulty of perma- 
nently securing the fluid, and then, admitting this to be effected, the probability 
of a decomposition of the glass by the fluid, &c. &c. I have, however, now the 
satisfaction to state, that the lens of my 3-inch telescope, filled August 5th 1827, 
continues in precisely its original state, no perceptible change having yet taken 
place in either the quantity or quality of the fluid, or in the transparency of 
the glass. 
As far as the above observations and remarks extend, therefore, it appears 
that the essential properties of the flint lens are supplied by the fluid. I beg 
now to state a few particulars in which the sulphuret of carbon has advantages 
which the glass has not : — these are, first, that in consequence of the very high 
dispersive power of this fluid, the correcting lens is placed so far behind the 
principal plate or crown lens, as to require to be only one half as much in 
diameter ; a highly important consideration in the construction of a very large 
telescope. 
Secondly, the combination is such as to give a focal power one and a half 
times the length of the tube, or, which is the same, the telescope may be 
reduced to two thirds the length of a glass telescope of the usual kind, without 
incurring a greater amount of spherical aberration in the front lens. 
Of the latter advantage, however, I have not ventured fully to avail myself in 
my 8-inch, because, as I knew the general opinion was against the success of the 
experiment, I was fearful of failing in the beginning by attempting too much. 
