The huge dimensions of such a powerful engine of research as the Hooker telescope are not in themselves a source of satisfaction to the astronomer, for they involve a decided increase in the labor of observation and entail very heavy expense, justifiable only in case important results, beyond the reach of other instruments, can be secured. The construction of a telescope of these dimensions was necessarily an experiment, for it was by no means certain, after the optical and mechanical difficulties had been overcome, that even the favorable atmosphere of California would be sufficiently tranquil to permit sharply defined celestial images to be obtained with so large an aperture. It is therefore important to learn what the telescope will actually accomplish under customary observing conditions. Fortunately we are able to measure the performance of the instrument with certainty. Close beside it on Mount Wilson stands the 60-inch reflector, of similar type, erected in 1908. The two telescopes can thus be rigorously compared under identical atmospheric conditions. The large mirror of the 100-inch telescope has an area about 2.8 times that of the 60-inch, and therefore receives nearly three times as much light from a star. Under atmospheric conditions perfect enough to allow all of this light to be concentrated in a point, it should be capable of recording on a photographic plate, with a given exposure, stars about one magnitude fainter than the faintest stars within reach of the 60-inch. The increased focal length, permitting such objects as the moon to be photographed on a larger scale, should also reveal smaller details of structure and render possible higher accuracy of measurement. Finally, the greater theoretical resolving power of the larger aperture, providing it can be utilized, should permit the separation of the members of close double stars beyond the range of the smaller instrument. CRITICAL TESTS The many tests already made indicate that the advantages expected of the new telescope will be realized in practice. The increased light-gathering power will mean the addition of many millions of stars to those already known. Spectroscopic observations now in regular progress have carried the range of these investigations far beyond the possibilities of the 60-inch telescope. A great class of red stars, for example, almost all the members of which were inaccessible to the 60-inch, are now being made the subject of special study. And in other fields of research equal advantages have been gained. The increase in the scale of the images over those given by the 60-inch telescope is illustrated by two photographs of the Ring Nebula in Lyra, reproduced in Fig. 18. The Great Nebula in Orion, photographed with the 100-inch telescope with a comparatively short exposure, sufficient to bring out the brighter regions, is reproduced in Fig. 2. It is interesting to compare this picture with the small-scale image of the same nebula shown in Fig. 1.