206 CARNEGIE INSTITUTION OF WASHINGTON. 



Photographs of spectra made with the 75-foot spectrograph of the 

 150-foot tower telescope include 124 of the disk for the sun's general 

 field, 80 of sun-spots, and 15 for miscellaneous purposes. 



Beginning May 20 and ending September 28, 1916, the sun was 

 photographed on 132 consecutive days. 



During the summer of 1917 there was much partial cloudiness; for 

 over five weeks some clouds foraied every day. 



STEREOGRAMS OF CALCIUM AND HYDROGEN FLOCCULI. 



The interesting results obtained last year by combining Ha images 

 of the disk for stereoscopic study have been extended, with special 

 reference to the elimination of false effects of reUef frequently encoun- 

 tered in such work. If two photographs of distant mountains, taken 

 from the extremities of a long base-line, are wrongly combined for 

 stereoscopic vision (left for right), the eyes easily see the lower moun- 

 tains in the foregTound as a deep cave, but reverse the peaks with 

 great difficulty. The tendency to persist in seeing such forms in their 

 normal relief doubtless explains the impossibility of reversing certain 

 stereograms of Ho. flocculi near the limb, where the time interval which 

 separates the two photographs is more than sufficient to give true 

 stereoscopic vision. 



It is obvious, however, that the surest results and the best sense of 

 relief will be obtained by combining images well separated in time, 

 provided that the changes in form of the floccuH and prominences 

 during the interval are not too serious. An admirable appearance of 

 sphericity of the solar image and high relief of the prominences on the 

 disk results from the stereoscopic combination of two Ha photographs 

 of small diameter, separated by an interval of 3 hours. Any false 

 effects caused in certain prominences by changes in form can be 

 detected by the use of various pairs of photographs made at different 

 times. 



The value of this method is obvious, especially in the study of prom- 

 inences on the solar disk. No one can doubt their character when 

 seen in this way, or make the mistake of attributing the dark flocculi 

 on Ha images to other causes. 



MAP OF THE SUN-SPOT SPECTRUM. 



As stated in the last annual report, our new map of the sun-spot 

 spectrum, on a scale of 1 cm. to the angstrom, was completed last year 

 by Mr. Ellerman for the region X 6000 to X 6450. During the winter 

 enlargements were made from additional negatives for the region 

 X 5400 to X 6000, and new negatives have now been obtained which 

 will permit the entire region from X 3800 to X 6600 to be included in 

 the map. 



