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[137] the comparative light of stars, or other heavenly bodies. Photometry, as it is called, is a specialty at the Harvard Observatory. Many photometers have been devised here, each adapted to some particular kind of work. A large variety of these is used with the large telescope. If none of the photometers in use seems to be exactly what is wanted for a particular piece of work, Professor Pickering or one of the staff invents a new one. The first one made was called A, and for a long time each new one received the succeeding letter, until the alphabet was exhausted. Now there is no special designation.

There is another instrument much used with the great telescope and that is the micrometer. This is a device for measuring very small distances and is much used in the study of double stars.

Before leaving the dome, we must read the list of donors printed on the walls. We ought also to step out on the balconies from which we have a fine view of Cambridge and surrounding towns by day, and by night an unobstructed view of the heavens.

Down stairs is another instrument of the very greatest importance,--the meridian circle. This is a telescope of fair size, large we should say if we had not just come from the fifteen inch equatorial. Its peculiarity is in the mounting. It turns on a rod pointing east and west, the ends of which are supported on heavy stone piers. It can therefore revolve freely in the plane of the meridian. Any star may be observed just as it crosses the meridian, but at no other time. Attached to the instrument is a large circle, very delicately graduated. The exact angle at which the telescope is turned to observe any star, is shown on this circle. Thus the star's height above the horizon is obtained, and

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