128 THE CANADIAN NATURALIST. [JuilC 



pressure, have enabled the conditions existing on the surface of 

 the suu to be imitated and watched in the laboratory. Mr. 

 Lockyer detects the presence of a spot by a general darkening of 

 the spectrum and the -widening of certain of the Frauenhofer 

 lines — phenomena which he attributes to a local increase in the 

 general and selective absorption of the chromosphere. The 

 Frauenhofer lines put on a sudden blackness and width in the 

 case of a spot with steep sides, but expand gradually in a shelving 

 one. This thickening of the absorption lines Lockyer and Frank- 

 land have proved, by experiments, to be due to varying pressure ; 

 and this variation in pressure they attribute to convection currents 

 in the chromosphere : " Suppose a hydrogen flame to be suddenly 

 projected from the sun in the direction of the earth, the waves of 

 light will be shortened, and the hydrogen lines of the spectrum be 

 shifted nearer the violet. If the flame travels from the earth the 

 waves will be lengthened, and the lines shifted nearer to the red 

 end of the spectrum. The line F undergoes strong contortions 

 when seen near the centre of the sun's disc. It is seen, in fact, 

 stopping short in one of the small spots, swelling out prior to 

 disappearance, invisible in a fecula between two small spots, 

 changed into a bright line, and widened out two or three times in 

 the very small spots, becoming bright near a spot, and expanding 

 over it on both sides, and so on. The Frauenhofer lines may 

 thus be looked upon as so many milestones, telling the rapidity of 

 the approach and downrush. Thanks to Angstrom's map of the 

 wave-length of the different parts of the spectrum, it is known 

 that the shifting of the F line the ten-millionth part of a mille- 

 meter nearer the violet, means a velocity of uprush to the eye of 

 38 miles per second. The observed alterations of wave-length is 

 such that twenty miles a second is very common." 



From this, I presume, we are to gather that Lockyer considers 

 that the same cyclone which whirls the chromosphere up into 

 space projects the heavier vapors of the photosphere into the 

 chromosphere, and thereby leaves a cavity in the photosphere 

 itself. This is filled by a downrush of the chromosphere, which 

 is, consequently, there much thicker than in the surrounding 

 region, and, therefore, more absorbent. 



Father Secchi's observations agree, in the main, with the 

 above. He remarks that when the slit of the spectroscope is 

 carried across a solar spot, the relative intensity, as well as the 

 lencrth of the spectral lines, changes. The spectrum is never 



