616 



SCIENCE 



[N. S. Vol. XXV. No. 642 



observer. As a matter of fact, however, a 

 spectroheliograpli capable of giving the 

 best results can easily be constructed of 

 materials ordinarily available in any ob- 

 servatory or physical laboratory. It is 

 sufficient, for many purposes, to photo- 

 graph only a narrow zone of the solar 

 image. In this case small lenses will suffice 

 for the collimator and camera, and small 

 prisms for the optical train. The lenses 

 and prisms may be mounted in wooden sup- 

 ports, on a wooden platform, rolling on 

 four steel balls in V-shaped tracks. The 

 motion of the instrument across the solar 

 image may easily be produced by a simple 

 screw, driven by a small electric motor. 

 Such a spectroheliograph was used to good 

 purpose at the Solar Observatory before 

 the permanent instrument was completed. 

 Brief mention may be made of some of 

 the numerous investigations possible with 

 such an instrument. It has recently been 

 found at the Solar Observatory that the 

 dark hydrogen flocculi, photographed near 

 the sun's limb, are slightly displaced with 

 reference to the corresponding calcium 

 flocculi. In general, they lie nearer the 

 limb. This probably indicates that the 

 absorbing hydrogen clouds are, on the av- 

 erage, at a higher level than the brilliant 

 calcium clouds. This subject deserves care- 

 ful investigation, extending over a consid- 

 erable portion of time. The type of spec- 

 troheliograph just referred to is as suitable 

 for the purpose as any instrument that can 

 be constructed. Another question, which 

 seems to be somewhat more difficult to solve, 

 is the actual difference in elevation of the 

 calcium flocculi, as photographed in the 

 Hi and H2 lines. Indeed, it is still a ques- 

 tion as to how important a part the dense 

 calcium vapor plays in determining the 

 form of the H^ flocculi. These objects 

 resemble the faculte so closely that they 

 appear practically identical with them, 



though slight differences, which are appar- 

 ently genuine, are occasionally found. 



Another method of investigating this 

 whole question of levels is afforded by the 

 spectroheliograph. It will be remembered 

 that when the level of sun-spots was last 

 under discussion, reference was made to 

 the relative radiation of the umbra and 

 neighboring photosphere, corresponding to 

 different distances from the center of the 

 sun. It was pointed out that when a spot 

 approaches the limb, its radiation decreases 

 less rapidly than that of the photosphere. 

 The natural conclusion was that the spot 

 lies at a higher level than the photosphere 

 and thereby escapes much of the absorption 

 produced by a comparatively thin layer of 

 absorbing matter. Recent observations at 

 Mt. Wilson have shown, however, that the 

 proportion of violet light in sun-spots is 

 miich smaller than in the case of the photo- 

 sphere. As it is known that the violet 

 rays undergo much more absorption near 

 the sun's limb than those of greater wave- 

 length, it is obvious that the light of the 

 spot would suffer less absorption, even if it 

 were at the same level as the photosphere. 

 Thus the only proper method of investi- 

 gating this question will be through the 

 use of monochromatic light. 



The spectroheliograph affords a simple 

 means of accomplishing this. It is only 

 necessary to make photographs of the spot 

 and adjoining photosphere, corresponding 

 to various distances from the sun's center. 

 The camera slit should be set on the con- 

 tinuous spectrum (not on a line), prefer- 

 ably in the violet or ultra-violet, since the 

 change of absorption would be most felt 

 in this region. In order to make photo- 

 graphic comparisons easily possible, the in- 

 tensity of the photosphere should be re- 

 duced to approximately the intensity of 

 the umbra, by means of a dark glass, 

 mounted over the collimator slit, but not 



