1880.] NEAV YOKK ACADEMY OF SCIENCES. 241 



tliis solar radiation, iJi a Inmp so to speak, gives fallacious re- 

 sults on account of atmospheric absorption; and that the neces- 

 sary correction compels us to increase our estimate of the 

 suii^s energy at least twenty per cent. In my own little book 

 upon the sun, published in 1S81, I had set the so-called solar 

 constant at twenty-five calorics per square metre per minute. It 

 is now certain thsit it must be put at least as high as thirty. 

 Professor Langley's investigations seem also to show another 

 remarkable fact — that we do not receive from the sun any at all 

 of the low-pitched, slowly pulsing waves, such as we get from 

 surfaces at or below the temperature of boiling water. Tlie 

 solar s])ectrum appears to be cut off abruptly at the lower end; 

 and this cutting off we know cannot have been effected in the 

 earth's atmosphere, because we receive from the moon in consid- 

 siderable quantity just this very sort of low-pitched rays. Lang- 

 ley finds them also abundant in the radiation of the electric arc, 

 so that we can hardly suppose them to be originally wanting in 

 the solar heat. It now looks as if we must admit that they have 

 been suppressed either in the atmosphere of the sun itself, or in 

 interplanetary space. Another striking conclusion first clearly 

 pointed out by Langley is that, if the sun's atmosphere were 

 removed, its light would be strongly blue. 



The Solar Surface and Spots. 



As regards the general make-up of the solar surface, I do not 

 think there has been any new fact of extreme importance brought 

 out within ten years. Janssen has, however, carried solar pho- 

 tography to higher excellence than ever attained before, and has 

 obtained plates that show the "granules'' and their grouping on 

 a scale previously unknown. He thinks that his plates prove a 

 peculiar constitution of the solar surface, consisting in collections 

 of clearly defined and rounded granules, separated by regions or 

 streaks where they are ill defined and elongated; and he calls tlie 

 phenomenon the "reseau photospherique,*' or photospheric net- 

 work. According to him the "net" remains approximately 

 constant for some minutes at a time, as shown by plates taken in 

 quick succession, but is subject to rapid and enormous changes 

 in periods exceeding a quarter of an hour or so. I find some 

 scepticism among high authorities as to the trustworthiness of 

 his conclusions. There are suggestions that the appearances 

 presented may be due to currents of air in the telescope tube and 

 at the surface of the sensitive plate; but I am disposed to think 

 he is right, for, on several occasions when the seeing has been 

 exceptionally fine, I have observed with my own eyes something 

 quite analogous, in our large telescope at Princeton. 



The spots have been carefully studied by several observers, by 



