MOUNT WILSON OBSERVATORY. 221 



arc" and other special light-sources, and to complete work already 

 well advanced on the "pole-effect" and its probable origin in the 

 electric field. Thus the researches of King, St. John, Anderson, and 

 Babcock can be developed on a scale commensurate wdth their import- 

 ance. The extension of our laboratory facilities \\dth this object in 

 view is the next considerable task of our construction division. 



Within the restricted limits of this report, it will not be feasible to 

 enter into all the details of our future scheme of research, which has 

 been prepared with the cooperation of the members of the Observatory 

 staff. But we may indicate the broad lines of some of the major 

 operations plainly indicated as both logical and necessary. In doing 

 so, we of course have especially in mind the new possibilities afforded 

 by the 100-inch Hooker telescope. 



This powerful instrument, whose construction, after many unavoid- 

 able delays, was still further impeded by the war, is now systemati- 

 cally at work. In another section of this report some of the tests to 

 which it has recently been subjected are described. So far as they 

 have gone, they indicate that the full measure of gain over the 60-inch 

 telescope, called for by the nearly threefold increase in light-gathering 

 power of the larger mirror, has been attained in practice. How exact- 

 ing are the conditions — optical, mechanical, and atmospheric — which 

 must be met to permit such a gain can be appreciated only by those who 

 have made a close comparative study of the performance of large 

 telescopes. Perrine long since pointed out the serious enlargem^ent of 

 the stellar image inseparably connected with increased focal length. 

 This, of course, is due to atmospheric disturbances, which are such as 

 to render considerations based on the size of the true diffraction image 

 of little meaning. In place of the minute spurious disk called for by 

 theory, we must substitute the "tremor-disk" (to employ Newall's 

 useful expression), which is of a very different order of magnitude. 

 Thus, there was ample room for apprehension that with an aperture 

 of 100 inches, and with the great equivalent focal length of 134 feet 

 attained at the Cassegrain focus of the Hooker telescope, the images of 

 stars might be too large, even under good atmospheric conditions, to 

 permit them to be usefully observed. Experience has shown, however, 

 that even at this focus the full theoretical gain in hght-gathering power 

 over the 60-inch reflector is realized in spectrographic work, and that 

 the demand on the atmosphere is not greater than can be met on Mount 

 Wilson during a great number of the nights of the year. 



This means a gain of a full magnitude in stellar spectroscopic work, 

 and it happens that this gain comes at a very critical point. For 

 example, as explained on page 262, the increase in available light per- 

 mits a general investigation to be undertaken of the long-period vari- 

 able (Md) stars, which are beyond the reach of the 60-inch with practi- 

 cable exposure times. These red stars, in addition to their peculiarities 



