Page:Astronomy for Everybody.djvu/95

One of the greatest advances in practical astronomy in our time has been brought about by photographing the heavenly bodies. This is so simple a process that the slowness of its introduction may seem curious. Back in the early '40's, Professor Draper, of New York, the well-known chemist, succeeded in making a daguerreotype of the moon. When the system of photography by our present process on a glass negative was invented, Professor Bond, of the Harvard Observatory, and Mr. L. M. Rutherford, an eminent astronomer of New York, both began to apply the art to the moon and stars. Mr. Rutherford brought his work to such perfection that his photographs of the Pleiades and other clusters of stars are still of great value in astronomy.

A photograph of the stars can be made by an ordinary camera if we only mount it like an equatorial telescope so that it shall follow the star in its diurnal motion. A very few minutes exposure will suffice to take a picture of more stars than can be seen by the naked eye; in fact, with a large camera, this will not require a minute. But what is generally used by the astronomer is a photographic telescope. Any ordinary telescope will serve the purpose, but in order to get the best results the object-glass of the telescope must be especially made to