The new _72-inch reflector_ at the Dominion Astrophysical Observatory, near Victoria, B. C., weighs nearly 4,500 pounds, and the moving parts about 35 tons.)] [Illustration: _Photo: H. J. Shepstone._ THE DOUBLE-SLIDE PLATE HOLDER ON YERKES 40-INCH REFRACTING TELESCOPE The smaller telescope at the top of the picture acts as a "finder"; the field of view of the large telescope is so restricted that it is difficult to recognise, as it were, the part of the heavens being surveyed. The smaller telescope takes in a larger area and enables the precise object to be examined to be easily selected.] [Illustration: MODERN DIRECT-READING SPECTROSCOPE (_By A. Hilger, Ltd._) The light is brought through one telescope, is split up by the prism, and the resulting spectrum is observed through the other telescope.] But there is a device whereby the power of these giant instruments, great as it is, can be still further heightened. That device is the simple one of allowing the photographic plate to take the place of the human eye. Nowadays an astronomer seldom spends the night with his eye glued to the great telescope. He puts a photographic plate there. The photographic plate has this advantage over the eye, that it builds up impressions. However long we stare at an object too faint to be seen, we shall never see it. With the photographic plate, however, faint impressions go on accumulating. As hour after hour passes, the star which was too faint to make a perceptible impression on the plate goes on affecting it until finally it makes an impression which can be made visible. In this way the photographic plate reveals to us phenomena in the heavens which cannot be seen even through the most powerful telescopes.