OF ARTS AND SCIENCES. 233 



ing for the exploration of tliat region, since it extends It, far more than 

 the prism can do; but, on making the attempt, was discouraged by the 

 ditficulty of getting rid of tlie more refrangible lines belonging to the 

 second spectrum. I had hoped to eliminate these by passing the ray 

 on its approach to the slit through a solution of the bichromate of 

 potash. But the bichromate in long exposures permits a suiiiciency 

 of the more refrangible rays to pass, to produce a maj-ked photographic 

 effect ; and hence I feared that any experiments supposed to prove the 

 existence of lines in the infra-red would be open to the criticism that 

 they, in reality, belonged to the more refrangible regions of the spec- 

 trum of the second order, and that a satisflictory examination of the 

 case would exclude the use of the grating and compel that of the 

 prism. With the prism I could not obtain clear evidence of the exist- 

 ence of more than three lines, or perhaps groups, and doubtful indi- 

 cations of a fourth. If in these examinations we go as far as wave 

 length 10,750, the limit of Captain Abney's map, we nearly reach the 

 line H" of the third spectrum. This would include all the innumera- 

 ble lines of spectrum 2, and even many of those of spectrum 3. In 

 such a vast multitude of lines, how would it be possible to identify 

 those that properly belonged to the first, and exclude those of the 

 second and third spectra ? Besides, do we not encounter the objec- 

 tion that this is altogether beyond the theoretical limit of the priamatic 

 spectrum ? 



This brings us to Captain Abney's recent researches, which, by the 

 aid of the grating, carry the investigation referred to the prismatic 

 spectrum -is far below the red as the red is below the yellow. They 

 are not to be regarded as an extension of exploration in the infra-red 

 region, — for they really do not carry us beyond my own observations 

 in 1842, — but as securing the resolution of these lines or bands into 

 their constituent elements. I had never regarded them as really single 

 lines. The breadth or massiveness of their photographs, too, plainly 

 suggests that they are composed of many associated ones. The princi- 

 ple of decreasing refrangibility with increasing wave length incapaci- 

 tates the prism from separating them, but the grating which spreads 

 them out according to their wave length reveals at once their compo- 

 site character. 



In Captain Abney's map, after leaving the red line A, we find three 

 groups: (1) ranging from about 8150 to 8350; (2) from 8930 to 

 9300 ; (3) from 9350 to 9800. These, admitting that the lines of the 

 subsequent grating spectra have been excluded, are then the resolution 

 of a, /3, y. 



