At the Lick Observatory photographs are made whenever the weather permits. A reflecting heliostat is used. This consists of a plain glass mirror mounted at the lowest extremity of an axis, which is parallel to the earth's. The mirror is so pivoted as to revolve about a second axis, perpendicular to the polar axis, which permits of its adjustment to the sun's declination, and when so adjusted is made to follow the diurnal motion of the sun by clock-work which revolves around the polar axis. In this way the earth's motion is neu tralized and the reflected rays kept in a horizontal direction.
The reflecting mirror consists of a circular glass plate 7 inches in diameter, slightly wedge-shaped, one surface of which is ground accurately flat This surface is figured with as much care as an objective, for upon its ability to reflect properly the rays which fall upon it, depends the per fection of the resulting image. This reflector is used plain, without any silver film to increase its reflective power, so that only a small proportion of the incident light (from 6 to io per cent), passes to the object glass to form the image.
The heliostat, objective and plate-holder are mounted on separate brick piers.
The focus of the instrument is inside the photographic dark-room at the other end of the telescope. At this focus is ar ranged a metal frame on a brick pier for holding the sensitive plate. On the wall, between the sensitive plate and the objective, is a vertical shutter working in guides, with a horizontal slit. The exposure is given by dropping this shut ter so that it falls several feet before the exposing slit comes to the sun's image. The width of the slit is adjustable, and thus the exposure can be varied through the necessary range, the exposure vary ing with the attitude of the sun. The exposure of winter time is about o".o04, and this is decreased with the northward motion of the sun to about in sum mer. The sun's disc is brightest at the centre, and toward the edge grows rapidly darker owing to the absorption of his atmosphere. The correct posure for the central portions is too short for the edge vice versa; hence, an exposure is adopted which gives the best general result and is one which slightly over-exposes the central portions and under-exposes the parts near the edge or limb.* Every eclipse expedition is fitted with complete apparatus for taking many views. In December, 1889, two expedi
tions were sent out, the one to Barbados, _ _ and the other to Loanda in West Attica, to watch and photograph the total solar eclipse of December 22d, to detect any possible change in the corona during the two hours and a half that elapse between totality at Loanda and at Barbados, and to determine the photometric intensity of the corona. Each observer took with him a reflecting telescope of 20 inches diameter fully equipped with arrangements for photographing. The focal length was only 45 inches, and the diaphragm diameter 15 inches ; the focal aperture was therefore f/3.
Probably the most photographed of celestial bodies is the moon, although little has been revealed that the eye cannot see. The largest number of portraits of this body has been made at the Lick Observatory. The great Lick telescope with its object glass of 36 inches diameter and 57 feet focus is a remarkable instrument, and when in front of this is attached a supplementary meniscus lens of 33 inches in diameter, the combi nation forms the largest photographic camera in the world, having a focal length of 47 feet inches, and one especially suitable for photographing the moon.
The sensitive plate is held at the principal focus in a compound slide-rest, shown in the accompany ing sketch (Fig. 41).* The frame AA is fixed in the tube at right angle to the optical axis. The frame D and everything above is movable in declination by the handle Decl. ; the frame BC and the eye piece G are movable in right ascension by the handle RA. The guiding eye-piece G' receives a beam from the object-glass, and the image of a star can be kept on its curvatures by suitably moving the handles. The wires are lighted by a small in candescent lamp. The plate-holder PI', shown in the cut, is 8 x io, the image of the moon is about 5 inches in diameter. I. and II. are curtains on spring rollers for quick exposures. The two handles and the end of G project several inches outside of the telescope tube. The whole apparatus can be rotated in position angle through 25 degrees on the ring BB.
With an aperture of 8 inches, or about /77 ir, the necessary exposure for a general effect was found to be about three seconds. Bright regions, however, require much less, and the darker parts four or five seconds. A series of negatives are, therefore, made with different exposures.