138 REPORTS OF INVESTIGATIONS AND PROJECTS. 



Direct Photography of the Sun. — Mr. Ellerman or Mr. Olmsted have 

 made direct photographs of the sun daily, with the Snow telescope, on a 

 scale of 6.7 inches to the diameter. As these negatives have proved very 

 useful in connection with the study of spectroheliograph plates, this series 

 of photographs will be continued. 



Work zvith the Spectroheliograph. — The 5-foot spectroheliograph has 

 continued to give excellent results and has required no modifications of any 

 kind. The daily series of photographs comprises negatives made with the 

 lines of calcium, hydrogen, and iron. 



I have also devised a simple attachment for the spectroheliograph, to be 

 used in a determination of the level of sun-spots. It will be remembered 

 that in the last discussion of this subject it was pointed out that sun-spots 

 are probably of higher level than the photosphere, since their heat radiation 

 falls off less rapidly than that of the adjoining photosphere when the spots 

 approach the limb. In our studies of spot spectra, however, we found that 

 the spot radiation contains a larger proportion of the less refrangible rays 

 than the photospheric radiation. It might therefore be expected that even if 

 the spots were at the level of the photosphere, and subject to the same absorp- 

 tion, they would fall off less rapidly in intensity near the limb. It is evident 

 that in order to test the question monochromatic light should be used. As 

 the spectroheliograph offers the simplest means of obtaining monochromatic 

 images, using light from the continuous spectrum at any desired wave- 

 length, it was decided to employ it in preference to bolometric or other heat- 

 measuring apparatus, particularly as it should permit smaller spots to be 

 investigated. There appears to be only one difficulty in the use of the instru- 

 ment for this purpose, arising from the fact that the umbra requires from six 

 to ten times the exposure needed for the photosphere (for yellow light), in 

 order to get approximately equal density on the plate. This can be easily 

 overcome by the use of a screen of dark glass, through which the photosphere 

 is photographed. Two small plates of dark glass are mounted on the colli- 

 mator slit of the spectroheliograph in such a way that the spot is included 

 between their beveled edges, while the light from the neighboring photo- 

 sphere is reduced in intensity by the glass. The distance between the plates 

 can be varied, so as to permit their use with spots of various sizes. The 

 intensity of the spot and that of the neighboring photosphere (reduced by a 

 known amount) are measured with a Hartmann photometer. It is hoped 

 that in this way any change in the relative radiation of spot and photosphere 

 can be determined with considerable accuracy. The photographs are being 

 made with the camera slit set on the continuous spectrum in a region free 

 from lines. This wave-length is chosen in preference to a point in the blue 

 or violet, since the effect of scattered light from the sun and sky would be 

 much more serious in the more refrangible region. 



