372 Mev. Mr. FoicelVs Historical Sketch [May, 



Under the former head we may enumerate several cases in 

 optics, in which a variation in the intensity of light takes place, 

 which it would be desirable to reduce to some detinite measure, 

 so as to establish the law of the phenomenon ; for instance, the 

 different degrees of light reflected from a plane surface at dift'e- 

 rent angles of incidence ; and the intensities of polarized light 

 reflected at different azimuths of the second glass in Malus's 

 apparatus. 



The importance of some mode of comparing the intensity of 

 lio-ht from the various natural sources, whether celestial or ter- 

 restrial, is also readily seen, towards a more complete knowledge 

 of the constitution of things. To compare in this way the 

 brightness of the fixed stars, as vi'ell as of the reflected light of 

 the planets, is of the highest interest to the astronomer; whilst 

 to obtain similar measures of the light emitted from incandescent 

 and burning bodies is not less important to the chemist and the 

 natural philosopher. 



In the more mixed departments of physical inquiiy, the con- 

 nexion between the illuminating power and the heating (or, 

 according to my principle, the latent heat of the light), in dif- 

 ferent cases, readily occurs as a point, to the determination of 

 which the possession of some standard of measurement would 

 be very essential. The phenomena of the solar spectrum are a 

 familiar instance where this sort of measurement is of pecuhar 

 interest. 



In relation to practical objects, the illuminating powers of the 

 light produced by the different forms of lamps, by the combus- 

 tion of different materials, and especially of the diifferent sorts of 

 gas, have excited much inquiry and controversy. And in rela- 

 tion to these, as well as the former topics, with all the boasted 

 improvements of science, we have still to confess that no accu- 

 rate and unexceptionable method of measurement or comparison 

 has yet been proposed, or put in practice. 



The interest which both the former scientific questions, and 

 especially these last practical ones, have excited of late, leads us 

 to hope that the subject may, before long, undergo so thorough 

 and well-instituted an examination as may produce some 

 method of obtaining at least an approximate measure of the rela- 

 lative intensities of light. 



Meanwhile it may neither be uninteresting in itself, nor, per- 

 haps, without some use in subserviency to such a design, to take 

 a brief and cursory survey of the progress which has been made 

 in this branch of science ; and to make a few remarks on the 

 principles of the various methods hitherto proposed, which may 

 possibly lead to some suggestions for their further improvement. 

 It does not appear that the idea of measuring the intensity of 

 light occurred to philosophers till at a comparatively recent 



