150 Cassegrane Telescope. 
‘¢ Although it has long been remarked, that the Galilean 
telescope gives a clearer and better defined image than those 
in which convex eye-glasses are employed, ihe cause seems 
never to have been suspected, but was always attributed to 
the thinness of the eye-glass, and not to the form, which is 
similar to the Cassegranian. If this hypothesis, founded 
on the result of the experiments, be correct, the loss of 
light in the Gregorian telescope originates ithe rays of 
light reflected from the surfaces of the concave specula 
meeting in their respective foci ; and being thus condensed 
and concentrated, the particles of light do really obstruct 
and impede each other : thus a great part is dissipated and 
lost, thereby occasioning that dimness so remarkable in 
the Gregorian’ telescope when compared with one on the 
Cassegranian principle of the same aperture. In the Cas- 
segranian form, the above inconvenience is avoided, the 
convex small mirror being placed withim the cone of rays 
teflected from the large speculum, in proportion to their 
focal length; adding this important advantage, they are 
capable of being made less than half the length of the Gre- 
gorian. The manner in which the quantity of light was 
proved is as follows :—An annulus was put in the end of 
the Cassegranian tube, on which annulus concentric circles 
were drawn at the distance of one-tenth of an inch from 
. “each other; a printed card placed at the distance of thirty- 
six yards; the concentric circles were then cut one by one 
away till the letters on the card appeared equally bright in the 
Cassegranian as in the Gregorian telescope of the same di- 
mensions which was placed by its side: the apertures were 
then carefully measured, and it was found that in the Casse- 
‘granian telescope two inches and seven-tenths gave as 
bright an image as the Gregorian, the aperture of which was 
three inches and eight-tenths. These specula were both 
cast at the same time, and the polish was equally good in 
one as inthe other. Still, it appeared a circumstance 
of such an extraordinary nature, that Major Kater (who 
was present at all the experiments) and myself almost 
doubted the evidence of our senses, till another trial com- 
pletely convinced us of the fact. Two telescopes were or- 
dered of the same dimensions, one Gregorian, the other 
Cassegranian. J took every possible care in the construc- 
tion of each ; and after having completed them, the experi- 
ment was tried exactly as before, and to our complete sa- 
tisfaction the result was exactly the same. Other experi- 
ments, the last of which was made this morning, furnish 
: ; . . of = at ? - - . proofs 
