Ai'KiL 19, 1907] 



SCIENCE 



615 



the -wooden support for the lens, which 

 serves at once for collimator and camera. 

 The angular aperture of this lens is, of 

 course, defined by that of the objective 

 which forms the solar image on the slit, 

 tut if possible its focal length should be 

 from ten to twenty feet. The rays, after 

 being rendered parallel by the lens, fall 

 upon a grating, which need not be larger 

 than a four-inch (a much smaller one 

 would do very useful work). The spectra 

 should be photographed in the second, 

 third or fourth order, so as to give sufficient 

 scale. 



"With such an instrument, new work of 

 •great value may be done. Even with a 

 very small solar image, a photographic 

 study of the solar rotation should yield re- 

 sults of great precision. Halm believes, 

 from his spectroscopic work, that the rota- 

 tion period varies with the solar activity. 

 This is yet to be confirmed, but the ques- 

 tion well deserves investigation. There is 

 some reason to think that the rotation 

 period is not the same for different sub- 

 stances in the reversing layer. The iron 

 lines, for example, may give values differ- 

 ent from those obtained with the carbon 

 lines. It is also interesting te inquire 

 whether the enhanced lines of an element 

 give the same period as the other lines in 

 its spectrum. 



Another interesting investigation, which 

 does not require a large solar image, is the 

 study of the radial velocity of the calcium 

 vapor in the flocculi. It is only necessary 

 to measure, with great precision, the wave- 

 lengths of the H2 and H^ lines, correspond- 

 ing to various points on the solar image. 

 In this way the rise and fall of the calcium 

 vapor in the flocculi can be ascertained. 

 To be of the most service, this investigation 

 should be carried on in conjunction with 

 some other study of the flocculi. 



The photographic study of sun-spot 



spectra offers a most promising oppor- 

 tunity. It is a very easy matter to photo- 

 graph spot spectra in such a way as to 

 record for study thousands of lines which 

 are beyond the reach of visual observation. 

 Nevertheless, this has been accomplished 

 only recently, simply because spectrographs 

 of suitable design have not previously been 

 applied in this work. At the Solar Ob- 

 servatory on Mt. Wilson it has been found 

 that, in general, the lines strengthened in 

 spot spectra are strengthened in the labora- 

 tory when the temperature of the vapor is 

 reduced, while the lines that are weakened 

 in sun-spots are weakened in the laboratory 

 under the same conditions. Thus it ap- 

 pears probable that the temperature of the 

 spot vapors is below that of the reversing 

 layer. This conclusion has been confirmed 

 by the discovery in the spot spectrum of 

 the flutings of titanium oxide. This com- 

 pound thus exists at the lower temperature 

 of the sun-spot, but is broken up into ti- 

 tanium and oxygen at the higher tempera- 

 ture of the reversing layer. The bearing 

 of this result upon stellar spectroscopy will 

 be seen when it is remembered that the 

 flutings of titanium oxide form the prin- 

 cipal feature of the spectrum of the third- 

 type stars. It has also been found that 

 Arcturus gives a spectrum resembling very 

 closely the spectrum of a sun-spot. A fur- 

 ther study of this question will require a 

 large number of observations of spot spec- 

 tra, with special reference to the question 

 of variations in temperature, as indicated 

 by variations in the relative intensity of 

 the spot lines. As already remarked, the 

 temperature of spots may also depend upon 

 their level, and this possibility must be 

 borne in mind. 



Work with the Spectroheliograph.— It is 

 perhaps commonly supposed that the spec- 

 troheliograph is necessarily an expensive 

 instrument, out of reach of the average 



