of a Solar Spectrum. 167 



media, endeavouring to get rid of all the disturbances which 

 arise through the absorptive action of glass by using as the 

 grating a polished surface of steel on which lines had been 

 ruled with a diamond, and employing a concave mirror 

 instead of an achromatic lens ; and though my results were 

 imperfect and incomplete, I saw enough to convince me that 

 it is absolutely necessary to employ a spectrum that has 

 been formed by reflection alone. (Phil. Mag., March 1857, 

 p. 155.) 



In 1871, M. Lamanski succeeded in detecting these lines 

 or bands by the aid of a thermomultiplier. He was not ade- 

 quately informed on what had already been done in the matter 

 in America ; for he says that, " with the exception of Foucault 

 and Fizeau in their well-known experiments on the inter- 

 ference of heat, no one as yet has made reference to these 

 lines." Nearly thirty years before the date of this memoir I 

 had published an engraving of them (Phil. Mag., May 

 1843). 



After I had discovered these three lines, I intended to use 

 the grating for the exploration of that region, since it extends 

 it far more than the prism can do — but on making the 

 attempt was discouraged by the difficulty of getting rid of 

 the 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 

 sufficiency of the more refrangible rays to pass, to produce a 

 marked 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 region of the spectrum of 

 the second order, and that a satisfactory 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 existence of more than three lines, or perhaps groups, 

 and doubtful indications 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 2 of the third 

 spectrum. This would include all the innumerable 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 Ave not encounter 

 the objection that this is altogether beyond the theoretical 

 limit of the prismatic spectrum ? 



