OF ARTS AND SCIENCES. 63 



protuberances which I have made at the Harvard College Observatory, 

 in which the limb of the sun is seen through the base of the protuber- 

 ances in front of it. In plate X, figure 3, there occurs a very striking 

 instance, where two small prominences are seen through a larger pro- 

 tuberance nearer the observer. 



During this period of general subsidence, the granulations appeared 

 to be smaller and farther apart than usual, and consequently the light- 

 gray colored background uj^on which they are seen projected was more 

 distinct, as it occupied more space than formerly. During this period, 

 the light-giving element would appear to have been less than usual. 



I am not aware that the phenomena of which I shall speak in this 

 communication' have been before observed; but I cannot speak posi- 

 tively on this point, owing perhaps to the somewhat confused nomen- 

 clature of solar physics. 



Ever since I have observed the sun with instruments of a large 

 aperture, I have noticed that the light-gray colored background seen 

 between the granulations is by no means uniform, as it is generally 

 stated to be. On the contrary, it is greatly and strikingly diversified. 

 Aside from the very small black dots called "pores," patches of a 

 darker gray are irregularly distributed all over the surface of the sun. 

 But partly owing to the effect of perspective, and partly on account 

 of the thicker strata of the chromospheric gases through which they 

 are necessarily seen near the limb, they disappear gradually as they 

 approach the border. 



These dark spots have been so remarkable during the present year, 

 and so conspicuous during the period of the greatest subsidence of the 

 chromosphere, that I have availed myself of every favorable opportu- 

 nity to study them. So strongly were they marked, that when one 

 had passed the field of view, it coukl be easily found again among 

 many others, even after the lapse of several hours. Of the most 

 striking and complicated, I have made sketches. 



In order to be able to count how many of these gray spots could be 

 seen in different heliographic latitudes, and also to estimate their area 

 with respect to the whole surface of the sun, Mr. W. A. Rogers, 

 Assistant at the Harvard College Observatory, liindly ruled for 

 me on glass a reticule of small squares. Though the problem is 

 apparently a simple one, it nevertheless presented many difficulties ; 

 partly owing to the minuteness and delicacy of these objects, partly 

 on account of the unsteadiness of the atmosphere, and partly to the 

 many defects caused by the great amount of heat concentrated at the 

 focus of the objective. However, the observations show clearly that, 



