POPULAR SCIENCE 291 



central meridian. Moreover, the displacements observed when 

 the spectroscope slit bisects a spot in a direction at right angles 

 to a line joining the spot and the centre of the sun's disc are 

 small. If the displacements were due to rotation in the spot, 

 this latter position of the slit should show them to the greatest 

 advantage. The only explanation to account for these dis- 

 placements in the spectrum lines over the spot is that they 

 are caused by radial movements in the vapours constituting 

 the spot from the centre outwards towards the penumbra. 

 This would accord with the radial structure of the filaments in 

 the penumbra of spots as observed visually. 



These interesting observations of Mr. Evershed have been 

 extended and amplified by Dr. St. John at the Mount Wilson 

 Solar Observatory. He observed the radial velocities in sun- 

 spots in the cases of 506 lines of the spot-spectrum, representing 

 27 chemical elements, and lying at levels extending from the 

 lowest that can be observed to the highest regions in the chromo- 

 sphere. The reversing layer is the name given to a shallow 

 stratum of vapours and gases some 500 miles in depth, which over- 

 lies the photosphere or brilliant light-giving surface of the sun . 

 It is in this region that the absorption which gives rise to the 

 dark lines in the solar spectrum takes place. When the moon 

 covers the sun in a total eclipse, the reversing layer at the edge 

 of the sun is isolated, and immediately, for a few seconds only, 

 all the dark lines of the solar spectrum become bright, the so- 

 called " flash spectrum " being produced. Above the reversing 

 layer lies the chromosphere, or atmosphere of hydrogen, some 

 3,600 miles in depth, according to the observations made at 

 Stonyhurst during twelve successive years. From the atmo- 

 sphere rise the flames of hydrogen and calcium and helium, 

 sometimes to enormous heights, called the prominences. From 

 eclipse observations, the mean height to which hydrogen rises 

 is about 4,500 miles, helium 3,500 miles, and calcium about 

 6,000 miles. The levels of hydrogen and calcium flocculi are 

 presumably still higher, possibly between 15,000 and 16,000 

 miles. These different vapours and gases are not arranged in 

 successive and isolated layers, like the coats of an onion, but 

 they interpenetrate one another, and rise from the photosphere 

 to different heights. Faint thin lines in the solar spectrum 

 would indicate a rise to a much lower level than would thick 

 black lines. Now, in the spectum of a sun-spot it is mainly the 



