ON THE KEW OBSERVATORY. 183 
the upper portion of E. This is employed to move it downwards (when the 
mouth is to be more nearly closed 65 65 must, of course, be released before e* 
is screwed downwards, and vice versd). 
A narrow vertical slit is cut in the lower lips of E, as shown in fig. 3, and 
a horizontal aperture of about 3 inches long, and about a quarter of an inch 
broad, (not shown) is cut through αἱ for the passage of light to e'. 
F is the slider-case for receiving the sliding-frame. 
7, a perfectly true ruler of brass attached vertically to αἱ by means of 
three screws passing through it, through three little pillars and through three 
oblong slits in a!, &c., which admit of its vertical adjustment. 
f? is a roller spring attached to a', and acting upon H laterally, pressing 
it gently against f°. 
{5 is a pair of similar springs acting upon H in front, and pressing a plate 
belonging to it (to be presently described) against E. 
G is the leris tube containing two groups of achromatic lenses (by Ross), 
and of curvature specially adapted to the purpose. The range of the image 
of the slit in the moveable shield is four times greater than the range of the 
slit itself (vide fig. 2. Plate I.). 
g' is apparatus of sliding plates, &c., for support and due centring of G. 
g* is apparatus of stud, pinion, milled-headed key, &c., for moving the 
rod g° which is attached to the stud at g*, and serves for adjustment to 
focus (of G). 
H is the sliding-frame suspended in F. Δ is a door closed by means of 
three little turn buckles, 45. Upon its interior side are fixed three springs, 
h', for retaining the Daguerreotype plate y (or a glass plate, if Talbotype 
paper is used) in its proper place. 
A? are the three friction rollers. 
_ 43, a hook with a little peg in it, which attaches it to a clock chain. 
h° is a brass plate capable of sliding freely in a groove in H. 
When both 2° and H rest on the bottom of E, A® covers entirely y (of 
course not touching it); and the height of 2° is such that when placed in F 
properly its upper edge always stands at about one-twentieth of an inch below 
the opening of the mouth at E, as shown by the dotted line; but when H is 
drawn upward (carrying V with it and leaving A® still resting on the bottom 
of E), portions of V are successively exposed to the action of light passing 
from the lamp (or daylight) through the slit in ὁ1, the lenses in G and the 
aperture 6}. 
At the upper end of A+ a narrow aperture and a piece of finely ground 
glass is placed opposite to it and above y, for the purpose of receiving the 
image (before the clock is started), and the microscope, f° (fig. 1. Plate I.), 
is used in examining the image on the ground glass for focus and colour. 
I is the pulley on the hour-arber (or barrel arbor) of the time-piece. Its 
diameter is somewhat less than 4 inches. It moves H upwards at the rate 
of an inch per hour: but a pulley of half that diameter may be substituted 
for it, and the time-scale thus diminished to half an inch pér hour if required. 
ἐϊ is the clock chain by which H is suspended from I; and 
2 is a counterpoise to H, &c. 
K (fig. 1. Plate I.) is the time-piece. 
R® (fig. 7.) is the back view of the lever and fork, &c. above mentioned, 
attached to an arbor passing through the clock plates, and furnished with a 
milled-headed nut (not shown in front), and by a spring and detent, , by 
means of which the fork can be made to stop or to release the pendulum at 
any given second. 
ΚΙ is the frame supporting K & F (vide Plate I. of former Report). 
R, brass tubular braces. 
