64 REPORTS ON INVESTIGATIONS AND PROJECTS. 



continuing the work special care will be taken (i) to obtain photo- 

 graphs with H, and with the continuous spectrum without change 

 of slit-width (in the temporary spectroheliograph a change of slit- 

 width has been necessary in order to compensate for the difference 

 in brightness) ; (2) to increase the contrast in photographs of the 

 faculae proper, so that the comparisons may be made nearer the 

 center of the disk, as well as near the limb. 



Mr. Evershed's alternative view that the H, photographs may be 

 due to H., light reflected from the slit jaws has been rendered un- 

 tenable. The beveled edges of the jaws in the temporary spectro- 

 heliograph are turned toward the plate, so as to reduce the possibility 

 of reflection ; but in any event the differences in form of H, and 

 H., images are such as to preclude this view. 



Hi photographs undoubtedly represent a lower level than H 2 pho- 

 tographs ; there is no question on this point. The doubt is whether 

 H, photographs show the faculae themselves or the calcium vapor 

 in or above the faculae, and whether the level represented depends 

 upon the position of the second slit on the Hj band. The Rumford 

 spectroheliograph results formed the basis of my working hypoth- 

 esis, but in some respects they were less suitable for this purpose 

 than the photographs recently taken on Mount Wilson. Now that 

 the 5-foot spectroheliograph is completed, it should be possible to 

 settle the matter in a short time. 



A description of this large instrument is reserved until the 

 completion of the tests now in progress. With its large aperture 

 (8 inches) and focal length (5 feet) ; the possibility of using a dis- 

 persion of from one to four prisms with an odd or even number of 

 reflections ; the wide range of speed afforded by the electric driving 

 mechanism ; the convenient means provided for focusing on the 

 solar image, setting on the spectral lines, and attaching slits of 

 various curvatures — this spectroheliograph may reasonably be ex- 

 pected to yield results surpassing those hitherto obtained. 



A spectroheliograph of 8 inches aperture and 30 feet focal length, 

 with three prisms of 50 , is now under construction. This instru- 

 ment is designed especially for the photography of limited regions 

 of the solar disk with Fraunhofer lines other than those of calcium 

 and hydrogen, including the lines affected in and near sun-spots. 



A method of measuring the heliographic latitude and longitude 

 of points on photographs of the solar disk by projection upon the 

 surface of a ruled globe has been successf ully employed by Mr. Fox 

 and myself at the Yerkes Observatory. A modified form of the 

 apparatus, which I have more recently devised, is now under con- 



