74 REPORTS ON INVESTIGATIONS AND PROJECTS. 



comprised holographic records of the radiation from various parts of the 

 solar disk, together with special studies of the radiation of sun-spots and 

 faculae. In the w^ork on the solar absorption, the prism is set so as to 

 bring a given wave-length upon the bolometer strip, and the solar image 

 is then allowed to move across the slit of the spectroscope, while the 

 photographic plate upon which the galvanometer deflection is recorded 

 is caused to move downward at a uniform rate. Since the height of 

 the slit is only 3 mm., the record thus obtained is a measure, for all points 

 along a solar diameter, of the radiation corresponding to the wave-length 

 employed. On account of the well-known fact that the absorption of the 

 solar atmosphere varies for different wave-lengths, the corresponding 

 curves are of different forms. Their reduction, by a method previously 

 employed by Mr. Abbot, gives a measure of the solar absorption. His 

 previous investigations have led him to the belief that the solar absorption 

 may show variations corresponding with the variations he has detected 

 in the value of the solar constant. For this reason it will probably be 

 advisable to continue routine observations of the character described, so 

 that the results may be compared with his contemporaneous measures of 

 the solar constant. 



The constant temperature room in the Snow telescope house, pre- 

 viously used for the large stellar spectrograph employed for long ex- 

 posures, has served admirably for the holographic work. The triangular 

 pier is large enough to carry all parts of the apparatus, for which it fur- 

 nishes a most suitable support. 



The holographic studies of sun-spots made by Mr. Palmer, with the 

 advice and cooperation of Mr. Abbot, have confirmed the result obtained 

 by Mr. Abbot last year, namely, that the radiation of sun-spots falls off 

 much more rapidly in the more refrangible region than does that of the 

 photosphere. In other words, sun-spots emit a relatively large propor- 

 tion of the less refrangible rays. It is extremely probable that this will 

 serve to account, in whole or in part, for the fact that the total radiation 

 of a sun-spot falls off less rapidly than that of the neighboring photo- 

 sphere as the spot is carried toward the limb by the sun's rotation. It 

 will be remembered that this phenomenon was the source of much dis- 

 cussion several years ago, and led many to the belief that sun-spots lie 

 at a sufficient height above the photosphere to escape a considerable part 

 of the general absorption. An explanation based upon the different 

 quality of spot and photosphere radiation seems more plausible, and is 

 capable of a rigorous test. It will only be necessary to employ mono- 

 chromatic light in measurements of the comparative radiation of spot 

 and photosphere at different distances from the limb. It is hoped that 

 these experiments may be tried, with the aid of the Snow telescope, 



