154 
DE. T. E. EOBHSTSON AND ME. T. GEUBB’S DESCEIPTION OP 
e by the gun-metal rings C and D (tig. 36). The frame carrying the prepared plate is 
attached by a convenient arrangement to the upper end of the tube/' and the exposing- 
shutter at the lower end, D, is worked by the lever and shaft, K, shown more fully in 
the two next figures (figs. 37 & 38). The adjustment for focus is made by the milled 
head cj (fig. 26), working the rack and pinion h, while a scale of divisions, s, is used to 
register the exact position of the focus. Fig. 37 is a plan, and fig. 38 an elevation of 
the exposing-shutter, each one-fourth of the full size. B, B, B (fig. 37) are, as before, 
the three arms of the tripod frame. C is the lower of the two gun-metal rings to which 
these are bolted. To this ring is attached, by two milled-headed screws E, E, the gun- 
metal frame D (shown hatched in the figure) which carries the shaft c in a pair of bear- 
ings f, f, to which is attached the thin sheet brass shutter F ; a is the shaft lettered K 
in fig. 36, which is acted upon from below by the lever K', fig. 36, and a pair of parallel 
cords passing down the side of the tube. This shaft, in turning through 90°, communi- 
cates a similar motion to the shaft c, which carries the shutter by the pair of cranks b, b 
and the connecting-rod d. As it is desirable to be enabled to open the shutter from 
either the right or left side to give a greater or less exposure to a particular side of the 
moon, this shutter is so arranged that, by taking out the two screws E, E, the frame 
D, which carries the shutter F, can be turned round 180° and fixed in that position; 
while the connecting-rod d , being disconnected by taking out the stud x, is thrown to 
the other side and attached to the spare crank V at the other end of the shaft c. At the 
same time the edge of the shutter can be adjusted parallel to the terminator of the moon 
by the rocking-piece G. The necessity, or at least the advantage of this arrangement, 
was shown in the experimental trials already mentioned ; for it was found that to obtain 
a tolerably uniform picture of the whole moon, many times more exposure was required 
for the parts near the terminator than for the bright edge. The apparatus includes a 
photographic micrometer similar to that described by Mr. W. De La Rue in his “ Ac- 
count of the Solar Eclipse of July 18, 1860,” Philosophical Transactions, vol. clii. page 
373, for tabulating the Photograms. 
The spectroscope has some modifications of the common construction, which are in- 
tended to make its adjustments more permanent. This is specially important in the 
present instance, because, as the telescope’s equivalent-focal length is very great, a minute 
shift of the collimator, the prisms, or the observing-telescope would make it extremely 
difficult to find the object’s spectrum. It is also desirable that the reading off the places 
of lines should not be embarrassed by continual changes of index error or prism adjust- 
ment, and that the instrument should be able to bear free handling, especially in the 
dark. Figures 39, 40, and 41 explain the construction, especially those parts which are 
new. Fig. 39 is half size, figs. 40 and 41 full size. In fig. 39, the part from A to B 
(shown in section), being of the usual construction, requires no description : the cylindric 
lens, of 6-inch focus, is of course achromatic and aplanatic. C is a strong cylindric box 
which holds the prism, and also serves to connect firmly the collimator and observing 
parts of the instrument. It effectually excludes false light, and protects the prism from 
