140 REPORTS OF INVEISTIGATIONS AND PROJECTS. 



gather determine, the records of the spectroheliograph. This subject will be 

 given the most careful consideration during the coming year, and efforts will 

 be made to ascertain, by experiments devised for the purpose, whether or not 

 we actually observe in the flocculi the effects of anomalous dispersion. 



Spectra of Sun-spots. — The inquiry into the cause of the characteristic 

 phenomena of sun-spot spectra, which was described in my last report, has 

 been continued during the year. It may be stated that the large amount of 

 evidence which has been brought together is decidedly favorable to the 

 hypothesis that reduced temperature of the spot vapors is the principal factor 

 in determining the relative intensities of the spectral lines, though no final 

 conclusions may be drawn until the possibility of accounting for the phenom- 

 ena on other grounds has been thoroughly tested. The pressure of other 

 work has hitherto prevented us from building an electric furnace capable of 

 giving much higher temperatures than the one previously employed, but it is 

 hoped that such a furnace, so devised as to be suitable also for investigations 

 on anomalous dispersion, will soon be ready for use. The investigation of 

 the cause of the spectroscopic phenomena of spots, which has hitherto been 

 confined mainly to the possible effects of temperature and pressure of the 

 vapors, will now be extended so as to include the influence, if any, of anom- 

 alous dispersion. 



Our detection of several of the flutings of titanium oxide in spot spectra, 

 and Professor Fowler's discovery that some of the flutings shown in our 

 photographic map of the spot spectrum are due to magnesium hydride, are 

 precisely what might be expected on the basis of our temperature hypothesis. 

 The titanium oxide flutings do not appear to be present in the solar spectrum 

 away from spots, but the lines of the magnesium hydride fluting seem to be 

 faintly apparent. If reduced temperature is to be considered as the prime 

 cause for the strengthening of these lines in spots, it becomes necessary to 

 explain how they can be produced, even faintly, in the solar spectrum. On 

 the one hand, it might be assumed that the temperature at some distance 

 above the photosphere is sufficiently low to permit hydrogen and magnesium 

 to combine. This is probably not the case, however, since bright lines of this 

 fluting have not, to my knowledge, been photographed in the flash-spectrum. 

 On the other hand, it might be assumed that the minute dark pores which dot 

 the surface of the sun between the granules consist of comparatively cool 

 vapors, which would give a spectrum analogous to that of spots. We have 

 no observational evidence to support this view ; in fact, the observed differ- 

 ences between the solar spectrum at the sun's center and near the limb are 

 perhaps to be considered as opposing it. For the present, therefore, we must 

 regard the hypothesis of reduced temperature in spots as purely tentative, 

 requiring the most rigorous tests that can be devised. 



