1826.] of Phoiometry, with Remark". 373 



period. Huygens threw out some hypothetical ideas relative to 

 this sort of inquiry ; but it would seem that Marie, a capuchiu 

 friar, was the first who treated the subject with any pretensioa 

 to accuracy; he published a small work about the close of the 

 17th century, but his first principles were erroneous. (See Leshe 

 on Heat, p. 404.) 



Bouguer published his elementary work on the subject in 1729; 

 an improved edition by Lacaille appeared in 1760. 



His method is founded on the principle that the tenuity of 

 light varies as the square of the distance from the source ; two 

 unequally luminous or illuminated objects are viewed nearly in 

 the same line, and the brighter removed till the impressions 

 appear equal ; the relative intensities of light are obtained from 

 the ratio of the squares of the distances. A modification of this 

 method was used by Sir W. Herschel in his experiments on the 

 quantities of light stopped by different media (Philos, Transac. 

 1800) ; and in various forms it has been employed in numerous 

 other experimental researches. 



Lambert, about 1760, published his treatise, where he intro- 

 duced the term Photometria. Prof. Leslie censures some of his 

 investigations ; while he allows him credit for genius and origin- 

 ality (Leslie, p. 405). He proposed the thermometrical principle; 

 but admitted the difference between its indications and the 

 degree of illuminating power. 



Count Ruraford proposed a new method by a comparison of 

 shadows ; for which an apparatus was contrived, to which he 

 gave the name of a photometer (Phil. Trans, vol. 84). The prin- 

 ciple was a modification of that of Bouguer; the shadows being 

 equahzed to the eye, the ratio of the squares of the distances of 

 the lights producing them gave the relative intensities. In allu- 

 sion to methods on these principles, Prof. Leslie observes, that 

 " machinery of such a complex nature is by no means entitled 

 to the name of a photometer ; each observation performed by it 

 is really a distinct process of experiment, and which requires 

 dexterity and skill in the operator." (406). 



But it is to be observed that all these njethods are merely 

 comparative ; we have no invariable standard of light, to which 

 other species or degrees of illumination are referred; no absolute 

 point of departure from which a scale of intensity commences. 



Mr. Ritchie, in a paper " on Leslie's Photometer, &.c." in the 

 Edinburgh Journal of Science, No. 4, maintains that the method 

 of shadows is defective when applied to light of ditterent colours, 

 in which case, he says, it is impossible to bring the shadows to 

 the same degree of density, and that a difference in colour will 

 always be perceptible. 



Another modification of the principle of comparing intensities 

 was adopted by Sir W. Herschel, for measuring the relative illu- 



