1826.] of Photometry, with Remarks. 375 



Wished fact, that if a body be exposed to the sun's rays, it will 

 in every possible case be found to indicate a measure of heat 

 exactly proportional to the quantity of light which it has absorbed, 

 and Suggests several experimental methods by which, he says, 

 the exact proportionahty may be shown, and that it depends on 

 darkness of colour. 



A full account of the principle, construction, and application 

 of his photometer, is given in ch. xix. A short sketch may be 

 acceptable to some readers, especially in reference to the objec- 

 tions presently to be noticed. 



This instrument is constructed with a view to measure the 

 degree of heat produced by the absorption of the solar rays, dis- 

 tinct from any other heating influence. 



On exposing a delicate blackened thermometer in a glass case 

 to the sun at different elevations, Prof. Leslie found the relative 

 effects correspond exactly with Bouguer's estimate of the inten- 

 sities of light at the same obliquities (p. 408). 



From his former experiments, he conceives it proved ih gene- 

 ral, that light and heat are convertible ; or that heat is only light 

 in a state of combination ; and upon this assumption, the absorp- 

 tion of heat by a black surface must be an exact measure of the 

 intensity of hght. 



The photometer, as is well known, is essentially a differential 

 thermometer, having one bulb of black enamel which absorbs 

 the rays : the other transparent, so that the temperature of the 

 surrounding air does not affect the instrument. 



The accumulation of heat in the bulb will of course be modi- 

 fied by the rate of its subsequent dissipation. This is increased 

 by any agitation in the air ; hence the whole is enclosed in a 

 thin glass case, which does away this source of inaccuracy 

 (p. 413). The temperature of the black ball continues to rise 

 till the accession of heat is balanced by the loss (p. 426) ; the 

 effect is unaltered by the size or thickness of the case (p. 435). 



Various suggestions for the application of this instrument iu 

 different questions of scientific and practical interest, are given 

 in chap. xx. : also in his " Description of Instruments," &c. 

 p. 11. 



The merits of this instrument have of late been much can- 

 vassed ; in particular, Mr. Ritchie, in his paper before referred 

 to (p. 322), has discussed several of the principal objections 

 which may be brought against it ; these are briefly the follow- 

 ing : — 1. The quantity of heat radiated from the black ball will 

 vary with the state of the sky, &c. ; the wind will also affect it 

 (by affecting the case). 2. Reflected light interferes with the 

 effect of the sun. 3. llluininatiiig power cannot be measured by 

 it, but only the quantity of light ; the former differs with the 

 colour of the light. 



In the same paper (note, p. 323), it is stated thai Pis. Turner 



