382 
15 inches’ aperture, and 12 feet focal 
length. Subsequently, however, this 
metal mirror was abandoned, and 
silvered glass, as suggested by M, 
Foucault, substituted. his latter, 
according to Steinheil’s experi- 
ments, reflects more than 90 per 
cent. of the light falling upon it, 
while speculum metal only returns 
63 per cent. A detailed account of 
this instrument, amply illustrated, 
is now being published by the 
Smithsonian Institution at Washing- 
ton, and therefore only a general 
idea of its pecularities will be given. 
As the telescope was intended es- 
pecially for photography, the follow- 
ing general principles were adopted. 
Ist. A reflector was, of course, pre- 
ferred to an achromatic object-glass, 
because all the rays falling upon it 
are reflected to the same focal plane, 
and there is not, as in the latter, one 
focus for distinct vision, and another 
for the photographically actinic 
rays, an inch distant perhaps. In the 
reflector a sensitive plate put where 
the image is seen to be most sharply 
defined, will be sure to give a good 
result. In the achromatic, on the 
contrary, the sensitive plate must 
be placed in a _ position which 
can only be found by tedious trials. 
2nd. Silvered glass was used instead 
of speculum metal, because it is 
lighter and more highly reflecting. 
Besides, if a reddish or yellowish 
film should accumulate on it—an 
accident liable to occur to either 
kind of reflector and seriously di- 
minishing the photographic power— 
it can either be repolished with a 
piece of buckskin—an operation ob- 
viously impossible in the case of a 
speculum metal—or the silver can be 
dissolved off with nitric acid, and a 
new film deposited on the glass con- 
cave. The glass which has been 
made accurately parabolic before the 
first silvering, is not changed in 
figure,the silver being only deposited 
in a layer s54555 Of an inch thick, 
and consequently, if carefully pre- 
pared, copying the glass below so 
closely that no error larger than a 
small fraction of that amount is 
possible. As the glass only serves 
as a basis or mould for the thin 
Notes and Correspondence. 
| April, 
sheet of silver, and is not penetrated 
by the light, its quality is a matter 
of but little moment, that which is 
used for skylights or light-openings 
in floors answering perfectly. 3rd. 
A mounting, presenting the greatest 
degree of steadiaess possible was 
necessary. For this purpose the 
telescope was supported at both 
ends, the lower one resting in a 
loop of wire rope. 4th. Instead of 
driving the whole mass of the in- 
strument by clockwork acting 
upon a polar axis, and thus being 
forced to move a weight of at least 
half-a-ton—the usual system in 
equatorials — only the sensitive 
plate and its frame, weighing an 
ounce, were caused to follow the 
moon or other object, the mass of 
the apparatus remaining perfectly 
at'rest. This idea is due to Lord 
Rosse. 5th. Instead of using a 
clock with wheelwork for a prime 
mover, a clepsydra was substituted. 
This consists of a heavy weight 
supported by the rod of a piston, 
which fits into a cylinder filled with 
water. At the bottom of the cylin- 
der a stopcock permits the water to 
flow out at a variable speed, de- 
pending on the amount of opening. 
The sensitive plate can thus easily 
be caused to coincide in rate with 
the moving object, and yet by a 
motion free from irregularity and 
tremor, 
The value of a silver reflector 
turns, of course, entirely upon the 
perfection of the glass concave on 
which the metallic film is to be de- 
posited. This must be of a para- 
bolic figure, so that spherical aber- 
ration may be completely corrected. 
A person is, however, content to 
take the utmost pains to produce it, 
because, once attained, the figure 
cannot be lost except by fracture, 
and the value does not diminish 
with time as in the case of a specu- 
lum. It never requires re-polishing. 
The best method of grinding and 
polishing the glass is by means of 
an apparatus that I have called a 
‘*Local-correcting Machine,” by 
which all the parts of the surface 
can be attacked in succession and 
reduced to the desired curvature, 
