24 ROUGH WAYS MADE SMOOTH. 



can be rendered discernible, remains to be seen. I must 

 confess my own hopes that the problem will ever be succes.- 

 fully dealt with are very slight, though not absolutely 

 evanescent. It seems to me barely possible that the pro- 

 blem might be successfully attacked in the following way. 

 Using a telescope of small size, for the larger the telescope 

 the fainter is the image (because of greater loss of light by 

 absorption), let the image of the sun be received in a small, 

 perfectly darkened camera attached to the eye-end of the 

 telescope. Now if the image of the sun were received on a 

 smooth white surface we know that the prominences and 

 the corona would not be visible. And again, if the part of 

 such a surface on which the image of the sun itself fell were 

 exactly removed, we know (the experiment has been tried 

 by Airy) that the prominences would not be seen on the 

 ring of white surface left after such excision. Still less, 

 then, would the much fainter image of the corona be seen. 

 But if this ring of white surface, illuminated in reality by 

 the sky, by the ring of prominences and sierra, and by the 

 corona, were examined through a battery of prisms (used 

 without a slit) adjusted to any one of the known prominence 

 tints, the ring of prominences and sierra would be seen in 

 that special tint. If the battery of prisms were sufficiently 

 effective, and the tint were one of the hydrogen tints pre- 

 ferably, perhaps, the red we might possibly be able to 

 trace the faint image of the corona in that tint. But we 

 should have a better chance with the green tint correspond- 

 ing to the spectral line 1474 KirchhofT. If the ring of 

 white surface were replaced by a ring of green surface, the 

 tint being as nearly that of 1474 Kirchhoff as possible, the 

 chance of seeing the coronal ring in that tint would be 

 somewhat increased ; and, still further, perhaps, if the field 

 of view were examined through green glass of the same tint. 

 It seems just possible that if prisms of triple height were 

 used, through which the rays were carried three times, by 

 an obvious modification of the usual arrangement for alter- 

 ing the level of the rays, thus giving a power of eighteen flint 



