614 



SCIENCE 



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



6 inches and a focal length of from 40 to 

 60 feet. In place of this objective, a con- 

 cave mirror, of similar aperture and focal 

 length, may be employed. This apparatus 

 will furnish the necessary means of form- 

 ing a fixed solar image, of large diameter, 

 within a laboratory, where accessory appa- 

 ratus can be mounted. Let us now con- 

 sider briefly some of the investigations that 

 can be undertaken. 



Direct Photography.— The routine pho- 

 tographic work, done under the direction 

 of the Greenwich Observatory, provides 

 ample material for the study of the posi- 

 tions and motions of sun-spots, but special 

 investigations may well be undertaken with 

 the aid of direct photographs. The impor- 

 tant thing in all solar work is not merely 

 to make observations of some single phe- 

 nomenon, but to carry on two or three 

 series of carefully correlated observations, 

 so designed as to throw light on one an- 

 other. For example, Mr. Maunder has 

 recently found that the rotation periods of 

 sun-spots in nearly the same latitude show 

 differences as great as those encountered 

 in passing from the equator to the highest 

 latitude in which the spots are found. The 

 cause of such differences may well be a 

 subject of most careful investigation. The 

 proper motions of spots, which are asso- 

 ciated with their period of development, 

 must be fully taken into account. We 

 might also make the hypothesis, merely for 

 the purpose of testing the question, that 

 the rotation period of a sun-spot depends 

 upon its level with respect to the photo- 

 sphere. For this reason it would be de- 

 sirable to investigate, in connection with 

 the study of the rotation, the question of 

 the level of sun-spots. A simple means of 

 doing this wiU be mentioned later. But it 

 may be added here that the question of 

 level raises other considerations, which 

 should not be left out of account. It is 



probably worth while to investigate photo- 

 graphically the old Wilsonian hypothesis, 

 since visual observations have proved so 

 discordant in attempts to determine the 

 relative widths of the preceding and fol- 

 lowing penumbra of spots at various dis- 

 tances from the center of the sun. As a 

 sun-spot is depressed below the level of the 

 surrounding faculse, the vexed question of 

 the visibility of the umbra near the limb 

 may depend upon whether faculte are pres- 

 ent or missing on the sides lying in the line 

 of sight. It is quite possible that the tem- 

 perature of the umbra may vary with its 

 distance above the photosphere. Thus cor- 

 relation between observations bearing on 

 spot level and observations of spot spectra 

 is desirable. 



Spectroscopy.— The spectroscopic study 

 of solar phenomena has been greatly re- 

 tarded, through delay in adoping suit- 

 able instruments. The short-focus spectro- 

 scopes attached to equatorial telescopes are 

 admirably adapted for visual observations, 

 but in photography their linear dispersion 

 is much too small to realize the full re- 

 solving power of the grating employed. 

 In laboratory work, on the contrary, while 

 the spectroscopes have been sufficiently 

 powerful, they have usually been of the 

 concave grating type, where astigmatism 

 interferes seriously with the study of solar 

 details, and the solar image on the slit of 

 the spectroscope has been so small that the 

 individual phenomena, in any event, could 

 not be separately distinguished. 



The construction of a powerful spectro- 

 graph of the Littrow type is an extremely 

 simple matter. A small slit, mounted on a 

 short metallic tube, is supported immedi- 

 ately above a long narrow photographic 

 plate. The wooden support for plate- 

 holder and slit rests on a pier and forms 

 the end of a long light tube of rectangular 

 section, which is closed at its other end by 



