384 
In the woodcut a is the telescope 
tube, b one of the trunnions on fric- 
tion rollers perforated for the eye- 
piece, ¢ one of the counterpoise 
levers, having a weight at the upper 
end and being attached to an axled 
at the lower end; ee’e” is a wire- 
rope going from the counterpoise to 
the lower end of the telescope ; ff 
another wire-rope which passes 
round a small drum connected with 
the winch g, and gives the observer 
standing on the observer’s platform 
i the power of moving the telescope 
in altitude; & the stairs going to 
the photographic room, //' the gal- 
lery that divides the Observatory 
into two stories, m the azimuth axis 
resting on the solid rock, and sus- 
tained at its upper end by the three 
lateral beams, nn’ (two only are 
shown). They also rest in cavities 
in the rock. The dome is seen in 
section at oo’, the dome-opening 
and shutter at p, the dome-arch at 
qq7. The dome-raising lever 7, with 
the fulcrum at s, is shown as it ap- 
pears when the dome is prepared 
for revolving, the axis ¢ carrying 
the whole weight. The part of the 
lever below the detent wu can be 
bent up out of the way, and held by 
a loop. 
Since the telescope has been com- 
pleted, and furnished with two para- 
bolic mirrors of 153 inches aperture, 
and 150 inches focal length, and one 
Herschelian mirror (that is, a con- 
cave of such figure that it can only 
bring oblique pencils to a focus free 
from aberration), Celestial Photo- 
graphy has been continually prose- 
cuted. About 1,500 lunar negatives 
have been taken. Old experience 
obtained from portrait and micro- 
scopic photography has proved to 
be of great service. At first the 
well-known processes were used, 
but it was soon found that some- 
thing more refined was needed, 
where the pictures are to be sub- 
mitted to magnifying powers of per- 
haps 25 times. Defects in collodion 
negatives that would, under ordi- 
nary circumstances, pass unnoticed, 
assume such prominence as greatly 
to diminish the beauty of the results. 
Notes and Correspondence. 
| April, 
These defects, pin-holes, coarse gra- 
nular appearance of the reduced 
silver, and other markings, were 
found to arise principally from the 
presence of nitrate of silver on 
the sensitive plate. It was ascer- 
tained that by washing the plate 
thoroughly before exposure they 
disappeared, or were very much 
ameliorated, and without any re- 
duction in sensitiveness. But for 
this washing operation pure water 
is needed, and hence the roof of the 
buildings was painted with a ground 
mineral compound that hardens to 
a stony consistence, and the water 
falling upon it was preserved in a 
leaden tank, which from long use 
for other purposes had become 
thickly coated with insoluble salts 
of lead, sulphates, &c. Whenever 
an inch of rain falls, a ton of water 
is collected, and the tank may he 
filled about 32 times in a year. 
The negatives produced at the 
focus of the reflector are on an ave- 
rage J;4;inches in diaineter. Many 
that have been made will bear to 
be enlarged to 2 feet, and one was 
taken September 3, 1863, at 4.30 
A.M., which has been increased to 
3 feet in diameter, the total magni- 
fying power used being about 380. 
In this photograph the moon may 
be said to be shown on a scale of 
60 miles to the inch. 
In the process of enlarging I have 
introduced one very important no- 
velty. Instead of employing an 
achromatic combination of lenses 
arranged as a solar camera, a con- 
cave mirror is used. It entirely gets 
rid of the difficulty of chromatic 
aberration, which is, as all pho- 
tographers know, one of the most 
serious obstacles to success, and, in 
addition, the magnified image lies 
in one plane, or there is what is 
termed a flat field. Every little de- 
tail of the original negative is per- 
fectly reproduced, and a_ 3-foot 
image is as sharp in one part as in 
another. The effect of portraits re- 
produced of life-size is very striking, 
and the resemblance to the indi- 
vidual singularly increased. In mag- 
nifying these lunar negatives, a 
