374 kev. Mr. Powell's Historical Sketch (May, 



minating power of the different primary rays (Phil. Trans. 1800), 

 but it, perhaps, was not susceptible of any great degree of 

 accuracy. 



The method employed by M. Frauenhofer in his researches on 

 the spectrum, for the same purpose, consisted in having a diago- 

 nal mirror placed at the focus of the eye-tube of a telescope so 

 that its edge bisected the field of view. Its surface being illu- 

 minated with variable intensity by a lamp having an adjustment 

 for that purpose, the light was equalized with that of the coloured 

 ray seen in the other half of the field : hence from the squares of 

 the distances of the lamp, the ratio of the intensities was deduced. 

 For a particular description, see Edin. Phil. Jour. No. 19, p. 32. 



I will not here enter xipon the objections which have been 

 raised to this method in the particular case to which it was 

 applied : they may be seen in an article on Optical Discoveries, 

 in the British Critic, No. I. Quarterly Series, p. 267. But those 

 objections will not apply in general to its application as a photo- 

 meter for other experimental purposes where the lights do not 

 differ much in colour ; the only difficulty in these cases would 

 be, that the absolute intensity of the light of the lamp cannot be 

 depended on as an invariable standard. 



Various modifications of the principle of comparing the 

 impression fromunequally illuminated surfaces are referred to by 

 several authors. Thus M. Biot mentions the method of compar- 

 ing the distances at which a book can be read under different 

 illuminations (Traite de Phys. vol. iv. p. 642, note) ; but all such 

 methods must be allowed to be open to considerable uncertainty. 



But we have now to turn our attention to methods of an entirely 

 different kind. The principle suggested by Lambert has been 

 already adverted to : he seemed to take for granted as the basis 

 of his mode of measurement, that the heat produced bi/, or at all 

 events accompani/irig the absorption of light, must be, generally 

 speaking, proportional to the intensity of light acting; but it is 

 far from clear whether he had ascertained how far this heating 

 effect might be considered a property of light, and whether he 

 was aware of any distinction to be drawn between the solar and 

 other sorts of light in respect to the heat accompanying or 

 belonging to them. At the same time (as I have already 

 observed) he clearly admitted the distinction between the inten- 

 siti/ or quautiti/ of light, and the illuminating power of a given 

 intensity of any particular light. 



In going upon the same thermometrical principle. Prof. Leslie 

 has taken the subject in a more extended point of view : he 

 connected the idea of such a measurement with the theory he 

 considered established from his experiments; according to 

 which the indication of heat would necessarily be a correct mea- 

 sure of the intensity of the rays. 



In his work on Heat (1804, p. 160), he mentions as an esta- 



