1824.] Mr. Potvell on Solar Light and Heat. 405 



absorption of light, and consequent heating effect, on the pig- 

 mentum nigrum, is equally unfavourable to distinctness of vision, 

 and the reason of this distinction may depend entirely on the 

 peculiarities of the optic nerve in regard to its susceptibility to 

 heat. 



(24.) In the experiment alluded to by Newton (Optics, book 1, 

 part 2, prop. 8), near the prism, white light occupies the central 

 part of the spectrum, which at greater distances disappears, and 

 yellow and green rays fill the space. According to his expla- 

 nation of this phenomenon, it follows that the illuminating effect 

 of the different parts of the spectrum must vary in their propor- 

 tions at different distances from the prism. 



(25.) The experiments of Sir W. Herschel (Phil. Trans. 1800) 

 on the relative powers of different substances for intercepting 

 light and heat, may be brought forward by some as a proof of 

 the separate existence of two such agents in the solar rays ; but 

 those experiments, as it will be evident on a more attentive 

 examination, do not bear at all upon this point. 



The powers of different sorts of glass, &c. to intercept the 

 sun's light, were measured by a comparison of the distances at 

 which equal illuminating effects were produced when an object 

 was viewed through the substance under trial, and by direct 

 vision. Their powers of intercepting heat were determined by 

 the comparative effects of the full solar rays on a naked thermo- 

 meter. 



The surface of the bulb of a thermometer, even when not 

 coated with any pigment, will still absorb a considerable portion 

 of the rays of light impinging on it, and will consequently be 

 affected by the inseparable heating power developed by the 

 absorption of those rays ; and the effect will be different accord- 

 ing to the colour of those luminous rays which the glass trans- 

 mits. Now the relations of the colour of the rays to heating, and 

 to illuminating effect, are by the experiments of the same author 

 proved to be extremely different ; so that the difference which 

 appears to exist in the power which the same substance has to 

 transmit light and heat is by no means a sufficient ground for 

 concluding the separate existence of two sets of rays. 



(26.) The consideration of the different laws followed by the 

 illuminating and heating powers of the differently coloured rays, 

 might lead us to doubt the close connexion between intensity of 

 light and of heating effect. Indeed this consideration must be 

 attended to in the attempt to compare by means of their heating- 

 effect on Leslie's photometer, light from different sources. In 

 these cases there is often a considerable difference in the preva- 

 lent colour of the rays, and, therefore, from this source alone, 

 some difference in their heating power is to be expected. 



In relation to the different intensities of light from the same 

 source, experiment seems to have shown that the closest propor- 

 tion is observed. Prof. Leslie has compared the indications of 



