378 Rev. Mr. PoweWs tJiitortcat Sketch [May^ 



** Availing myself (observes Mr. R.) of the ingenious remark 

 of Mr. Herschel, I have been enabled to obviate the objections 

 to which Mr. Leslie's photometer is obviously liable ; and can 

 now employ it with considerable accuracy as a measurer of the 

 isun's radiation" (p. 107). 



With the view of, in some measure, obviating the objection 

 against Leslie's photometer, Mr. R. (in a paper read before the 

 Royal Society in 1825) proposed coating the interior of the bulb 

 with a black substance. This improvement, however, is super- 

 seded by his own photometer, an account of which is given in 

 the Phil. Trans. 1825, Part I ; and some remarks made upon it 

 in the Edin. Jour, of Science, No. 4, p. 339. These remarks are 

 particularly deserving attention, in reference to the practical 

 object of comparing the relative illuminating powers of different 

 species of gas. 



This instrument was, in the first instance, invented upon prin- 

 ciples deduced from the author's theory, in which, with Prof. 

 Leslie, he supposes light to be merely caloric moving with great 

 velocity ; and that heat is developed in black substances by a 

 conversion of light into heat : however, without entering upon 

 theoretical discussion, the construction and action of the instru- 

 ment are of a nature calculated to avoid the errors just alluded 

 to. it consists of two air-tight chambers connected by a bent 

 tube, containing some coloured liquid, having their outer sides of 

 glass, and within each a diaphragm of black paper. The light 

 admitted through the glass is absorbed by the black paper, and 

 there giving out its latent heat, the temperature of the enclosed 

 air is raised. Two lights placed on opposite sides of the instru- 

 ment are compared by measuring the respective distances from 

 the instrument necessary to keep the interposed liquid stationary. 

 This instrument possesses extreme sensibility ; it is affected by 

 the light of a candle at 20 or 30 feet distance. The author has, 

 however, candidly acknowledged the inapplicability of any such 

 method for comparing the illuminating powers of light of differ- 

 ent colour and quality ; and in his paper (above quoted), he con- 

 cludes by saying, that " this celebrated question which has of 

 late agitated not only the philosophical, but even the commer- 

 cial world, has not yet received a solution sufficiently accurate 

 to command the assent not only of the impartial observer, but 

 even that of rival companies." (p. 325.) 



All the varieties of what has been termed the photometer, 

 upon the principle of the heat developed by a black surface 

 absorbing the rays from a luminous body, are founded on the 

 assumption that the heat thus produced is exactly proportional 

 to the intensity of light incident; but although this heating 

 effect has been proved to be associated with the light in the most 

 intimate manner, in a way which I have proved to be strictly 

 analogous to latent heat, it has not been shown by experiment. 



