65 



encountered. One is in the proper time to expose the 

 plate. When a photographer is taking a picture of a 

 room he exposes his plate several minutes; if he-is pho- 

 tographing a landscape scene in bright sunlight, a few 

 seconds he finds to be sufficient. For the sun, the short- 

 est exposure possible is required. The best photograph 

 of the sun yet obtained was made with an exposure of 

 ?uW of a second. In an eclipse, since the brightness is 

 much greater close to the sun than farther away from it, 

 a short exposure of a second or two will give the solar 

 prominences but no corona, while a long exposure will 

 represent more of the corona but the inner portions will 

 be -overexposed and therefore lose their details. This is 

 illustrated by two views, one of the eclipse of 1860, in 

 which the plate was exposed two seconds, and which shows 

 the prominences only, and the other Dr. Draper's photo- 

 graph of the last eclipse made with an exposure of 160 

 seconds. It was hoped by so long an exposure that the 

 coronal streamers might be photographed, which has 

 never yet been done. Indeed, from their absence on the 

 photographs it has been argued that they are unreal. The 

 picture does not show them, but it exhibits in the extreme 

 portions traces of a radiated structure. 



Another reason why reliance cannot be placed upon 

 photographs alone is that all rays of light are not chemi- 

 cally active. It was shown very graphically by Prof. 

 Osbun a few evenings ago that the actinic rays those 

 that are chemically active in developing the picture are 

 confined to the blue and violet and ultra-violet portions 

 of the spectrum, the last named not being visible to the 

 eye. Red and yellow rays affect the eye but not the sen- 

 sitized plate. Hence a photograph does not give a per- 

 fect representation, since it may omit peculiarities which 

 the eye discerns or add others invisible to the eye. (A 



