288 SCIENCE PROGRESS 



like that of the sun's photosphere. Moreover, in the variable 

 star Mira Ceti, which belongs to Type III., the titanium 

 oxide bands are much weakened at brighter maxima of the 

 star's light ; and the photosphere of the star is probably hotter 

 at such maxima, as the hydrogen bright-line spectrum shows 

 the blue radiation H/3, which is not seen when the titanium 

 oxide bands are more prominent. 



Again it is generally supposed, though it has not been 

 proved, that the electric spark represents a higher temperature 

 than that of the arc, and especially of the flame of the arc. The 

 lines that are widened in sun-spots are those which appear in 

 the arc flame, while those weakened are the so-called enhanced 

 lines which appear in the electric spark spectrum of an element. 

 But the argument is inconclusive, as some experiments of Dr. 

 Hemsalech show the enhanced lines in a flame at a lower tem- 

 perature than the flame which gives the arc lines. It may be 

 doubted whether these differences of behaviour in the lines of 

 an element in arc and spark are due at all to temperature, but 

 rather to a difference in the condition of electrical excitation. 

 Even in experiments with the electric furnace, the heating is 

 produced by electrical means, and the resulting spectra of the 

 metals heated in the graphite chamber may be conditioned, as 

 in the arc itself, by the mode of the production of the heat. On 

 the whole, however, opinion at present tends to the view that 

 the vapours in sun-spots are at a reduced temperature, being 

 at a temperature of about 3,500 degrees on the absolute Centi- 

 grade scale, while that of the photosphere is at a temperature 

 of 6,000 degrees on the same scale. Hence sun-spots, though 

 intensely bright in themselves, appear to be dark, as the cooler 

 vapours constituting them absorb the more intense radiations 

 of the sun's photosphere. 



The spectra of sun-spots, as observed at Stonyhurst for 

 more than thirty years, are substantially always the same, 

 the same lines being always picked out for widening, or weaken- 

 ing, or obliteration, and for the other effects to which lines are 

 subjected in the spot spectra, and also to the same degree, except, 

 perhaps, that the intenser iron lines are more apparent in 

 the minimum than in the maximum period of sun-spots. 



The discovery of the magnetic field in sun-spots, made by 

 Prof. Hale with the magnificent and highly dispersive apparatus 

 at Mount Wilson, is one of the most brilliant and important 



