AMERICAN 



JOURNAL OF SCIENCE AND ARTS. 



[THIKD SERIES.] 



Art. XIX. — On the Veiled Solar Spots :* by L. Trouvelot. 



[Read before the American Academy by WiUiam A. Rogers, Oct. 12, IStS.] 



It is now pretty well established that the visible surface of 

 the sun is a gaseous envelope called ''the chromosphere;" 

 mainly composed of incandescent hydrogen gas, with which are 

 occasionally associated some metallic vapors, usually occupying 

 the lower strata. To all appearances, the granulations called 

 " rice grains," the faculee and the protuberances, are phenomena 

 belonging to the chromosphere ; in fact they are the chromo- 

 sphere itself seen under the particular forms and aspects pecu- 

 liar to it. 'Ordinarily this envelope has a thickness of 10'' or 

 15". This thickness, however, is by no means constant, vary- 

 ing from day to day within certain narrow limits. 



At no time since I have observed the sun have I seen the 

 chromosphere so thin and shallow as during the present year, 

 and especially between June 10 and August 18. I had before 

 quite often observed local depressions and upheavals of the 

 chromosphere, sometimes extending over large surfaces, but I 

 had never before observed such a general subsidence. 



So thin was the chromosphere during this period that it was 

 sometimes very difficult to obtain its spectrum by placing the 

 slit of the spectroscope tangent to the limb of the sun. This 

 was especially the case on the afternoon of August 9. 



This unusual thinness of the chromosphere could be easily 

 recognized without the assistance of the spectroscope. Indeed, 



telescope, as, with it, the structure of the photosphere, lying as 

 * From the Proceedings of the American Academy. 

 A.M. Jour. Sot.— Third Series, Vol. XI, No. 63.~Ma»ch, 1S76 



