j\lr. J. N. Lockyer's Spectroscopic Observations of the Sun. 543 



a screen on which the image of the sun was received ; in this screen 

 there was also a fine slit corresponding to that of the spectroscope. 



By this method it is possible to observe at one time the spectra 

 of the umbra of a spot and of the adjoining photosphere or penum- 

 bra ; unfortunately, however, favourable conditions of spot (i. e. as , 

 to size, position on the disk, and absence "of cloudy stratum"), 

 atmosphere, and instrument are rarely coincident. The conditions 

 were by no means all I could have desired when my first observa- 

 tions were made ; and, owing to the recent absence of spots, I 

 have had no opportunities of repeating my observations. Hence I 

 should have hesitated still longer to lay them before the Royal So- 

 ciety had not M. Faye again recently called attention to the subject. 



On turning the telescope and spectrum-apparatus, driven by clock- 

 work, on to the sun at the date mentioned, in such a manner that 

 the centre of the umbra of the small spot then visible fell on the 

 middle of the slit in the screen, which, like the corresponding one in 

 the spectroscope, was longer than the diameter of the umbra, the 

 solar spectrum was observed in the field of view of the spectro- 

 scope with its central portion (corresponding to the diameter of the 

 umbra falling on the slit) greatly enfeebled in brilliancy. 



All the absorption-bands, however, visible in the spectrum of the 

 photosphere, above and below, were visible in the spectrum of the 

 spot ; they, moreover, appeared thicker where they crossed the spot- 

 spectrum. 



I was unable to detect the slightest indication of any bright 

 bands, although the spectrum was sufficiently feeble, I think, to have 

 rendered them unmistakeably visible had there been any. 



Should these observations be confirmed by observations of a larger 

 spot free from "cloudy stratum," it will follow, not only that the 

 phenomena presented by a sun-spot are not due to radiation from 

 such a source as that indicated by M. Faye, but that we have in this 

 absorption -hypothesis a complete or partial solution of the problem 

 which has withstood so many attacks. 



The dispersive power of the spectroscope employed was not suf- 

 ficient to enable me to determine whether the decreased brilliancy 

 of the spot-spectrum was due in any measure to a greater number 

 of bands of absorption, nor could I prove whether the thickness 

 of the bands in the spot-spectrum, as compared with their thickness 

 in the photosphere-spectrum, was real or apparent only*. 



On these points, among others, I shall hope, if permitted, to lay 

 the results of future observations before the Royal Society. Seeing 

 that spectrum analysis has already been applied to the stars with 

 such success, it is not too much to think that an attentive and 

 detailed spectroscopic examination of the sun's surface may bring 

 us much knowledge bearing on the physical constitution of that 

 luminary. For instance, if the theory of absorption be true, we may 

 suppose that in a deep spot rays might be absorbed which would 

 escape absorption in the higher strata of the atmosphere ; hence 

 also the darkness of aline may depend somewhat on the depth of the 



* Irradiation would cause bands of the same thickness to appear thinnest in 

 the more brilliant spectrum. 



