170 L. Trouvelot— Veiled Solar Spots. 



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 hundreds of times ; as is abundantly proved by the 

 numerous drawings of protuberances which I have made at the 

 Harvard Cbllege Observatory, in which the limb of the sun is 

 seen through the base of the protuberances in front of it. In 

 plate X, figure 3, there occurs a very striking instance, where 

 two small prominences are seen through a large protuberance 

 nearer the observer. 



During this period of general subsidence, the granulations 

 appeared to be smaller and farther apart than usual, and conse- 

 quently the light-gray colored background upon 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 can- 

 not speak positively on this point, owing perhaps to the some- 

 what confused nomenclature of solar physics. 



Ever since I have observed the sun with instruments of a 

 large aperture, I have noticed that the light-gray colored back- 

 ground 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 grav 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 neces- 

 sarily seen near the limb, they disappear gradually as they ap- 

 proach the border. 



These dark spots have been so remarkable during the present 

 year, and so conspicuous during the period of the greatest sub- 

 sidence of the chromosphere, that I have availed myself of 

 every favorable opportunity to study them. So strongly were 

 they marked that when one had passed the field of view, it 

 could 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, kindly ruled for me on glass a reticule of 

 small squares. Though the problem is apparently a simple 



