of Heat in the Prismatic Spectrum. 453 



lens*, he thought it more probable that the invisible rays 

 might be rendered visible by concentration, than that the lio-ht 

 existing there should be the cause of the heat. The convictTon 

 that the solar light was confined within the limits marked by 

 Newton, and that therefore no hght could be found beyond 

 but such as came there by accident and irregular dispersion, 

 may have induced Herschel to leave, in his investigations 

 of the illuminating power of prismatic colours, the space be- 

 yond them unexjilored ; at least we find none respecting it in 

 his rnicroscopical investigations. Whether therefore there was 

 any light there, and what were its effects, remained undecided. 



We may consider as one of the most important arguments 

 in support of invisible heating rays, the result of Herschel's 

 18th experimentf : I must therefore say something more upon 

 that. Herschel used in that experiment a lens of 9 inches in 

 diaaneter half covered with paste ; with which he received all 

 the prismatic colours on the covered part, and allowed all the 

 light, at a distance of l-5th of an inch from the limit of red, to 

 fall through that which was left open, and he assures us that 

 no trace of light or of colour was visible on the bulb of the 

 thermometer. That he perceived no light, no one will doubt : 

 but may it not have happened that it escaped his observation 

 perhaps by falling on a black bulb ; or that his eye, having 

 just been exposed to a strong light, was then less acute ? If 

 Newton, with an opening in the shutter of l-3rd of an inch, could 

 recognise this glimmering with the naked eye at a distance of 

 l-4th and l-3rd of an inch beyond the limit of the spectrum, 

 how much more light must there have been when Heischel 

 made use of a considerably broad prism quite uncovered ? I 

 have found, under similar circumstances, light at much greater 

 distances. 



Herschel mentions also a great number of experiments with 

 coloured glasses, in confirmation of his theory of heating rays. 

 But, however interesting the facts they contain, they still 

 seem to me to decide nothing concerning the main point in 

 dispute. To consider those experiments here would lead us 

 too far ; perhaps I may find another occasion to return to them. 

 I will only observe generally, that investigations of the effects 

 of coloured glasses, or on coloured light in general, will always 

 give unsatisfactory, and indeed confusing, results, as long as 

 the jxjlar contrast in coloured light is not taken into con- 

 sideration. The influence of this contrast extends to all the 

 functions of light; it is different in every one: no effect can 

 therefore serve as a criterion for the other, — that upon the eye 



* Phil. Trans. 1800, p. .'51". t Ibid. 



