62 PROCEEDINGS OF THE AMERICAN ACADEMY 



III. 



ON THE VEILED SOLAR SPOTS. 



By L. Trouvelot. 



Read by "William A. Rogers, Oct. 12, 1875. 



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 faculoe and the 

 protuberances, are phenomena belonging to the chromosphere ; in fact, 

 they are the chromosphere itself seen under the particular forms and 

 aspects peculiar to it. Ordinarily this envelope has a thickness of 10" 

 or 15". This thickness, however, is by no means constant, varying 

 from day to day within certain narrow limits. 



At no time since I have observed the sun, have I seen the chro- 

 mosphere so tliin 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 ex- 

 tending over lai'ge surfaces, but I had never before observed such a 

 general subsidence. 



So thin was the chromosphere during this period, that it was some- 

 times 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, the phenomenon 

 was even more interesting seen through the telescope, as, with it, the 

 structure of the photosphere, lying as it does under the envelope of 

 the chromosphere, could be better seen through the thin veil formed 

 by the greatly attenuated chromospheric gases. 



That the gases forming the chromosphere are sometimes thin enough 

 to become transparent is a phenomenon which I have observed hun- 

 dreds of times ; as is abundantly proved by the numerous drawings of 



