At the Observatory.

In addition to the continued observations of the new planet "Eros," carried on by Professor Pickering and Professor Wendell, at the Observatory, there is an unusually large amount of photographic work now being done by the department in intricate astronomical research.

Five instruments are now being used every night from sunset to sunrise to take photographs of different parts of the sky. During the day these plates are developed in the observatory dark room, by a mechanical method, recently devised by a member of the department. Twelve plates are placed at one time in a brass frame, which is kept in vertical motion by the action of a large pendulum. As the object is to get the faintest details, rather than the artistic effect of lights and shadows all plates are treated as if under exposed. In this way twenty-four plates may be developed simultaneously without requiring any constant attention. Previously this same work required the attention of several men, during a long period of time. After the plates are developed they are labelled, dated, catalogued, and after a second inspection, they are finally stored in the fire-proof photographic library. During the last year 12,000 pictures were taken in this way, and the library now contains nearly 100,000 phot graphs, many of which contain impressions of nearly 500,000 different stars. These pictures furnish a complete map of the sky for the past ten years. By means of these photographs the actions of a single planet can be traced to a period previous even to its discovery.

All the telescopes used in carrying on this work are operated in small sheds placed in different parts of the grounds, but each is controlled by a central electrical station. By the use of a switch board one man can operate from this station as many of the instruments as he may desire. In order that the motion of the earth may not effect the relative position of the photographic telescope and the object, the instrument is moved on an axis parallel to the earth and regulated by clock work in such a way that the distance the earth moves to the East is neutralized by the motion of the telescope in the opposite direction. By the eight inch Draper telescope the entire sky is photographed several times a year; the Cook lens takes a photograph of the sky every two weeks; and the photometer takes a similar photograph, with a short exposure every night.

There are two other stationary instruments. Over the end of one a prism is placed, which breaks up the light of the stars so that by the study of the rays the composition and character of any one star may be accurately determined. The second instrument is regulated by two ordinary alarm clocks, which open and close the shutter at any time the operator may wish. Every night this telescope is pointed directly at the north star, and after an exposure of twelve hours a semi-circle is thus traced on the plate. By this device an impression on the plate is made both of clouds and stars and a record of the clearness or cloudiness of the atmosphere is obtained, which is valuable for astronomical reference.

In addition to these photographs now being taken in Cambridge, a large number of duplicate pictures are also being taken at the station in Peru. The Bruce telescope there in use is the largest photographic telescope in the world. These photographs are also to be preserved at the Cambridge astronomical library and by a comparison of the two it is hoped that, interesting and valuable conclusions will be reached.