1 8 Experiments on the Relative Intensities 



tensity of the standard light might be discovered and 

 compensated ; but upon making the experiment I 

 found, what indeed a Httle patient reflection would 

 have enabled me to foresee, that the apparent density 

 of the two equal shadows corresponding to the lights 

 compared with a painted scale of shades, exposed in the 

 same light, is ever the same, however the intensity of 

 the rays at the surface upon which those shadows are 

 projected may var3^ 



There is, however, another method by which I 

 think it probable that the standard lamp might be 

 adjusted with the requisite degree of precision. It 

 appears, from a considerable number of experiments, 

 of which I shall hereafter give a more particular ac- 

 count, that the quantity of light emitted by a lamp on 

 any given construction, which burns with a clear flame 

 and without smoke, is in all cases as the quantity of oil 

 consumed. If therefore the standard lamp be so ad- 

 justed as alwa3'-s to consume a certain given quantity of 

 oil in a given time, there is much reason to suppose that 

 it may then be depended on as a just standard of light. 



In order to abridge the calculations necessary in 

 these inquiries, it will always be advantageous to place 

 the standard lamp at the distance of loo inches from 

 the photometer, and to assume the intensity of its light 

 at its source equal to unity. In this case (calling this 

 standard light A, the intensity of the light at its 

 source = .r = i, and the distance of the lamp from 

 the field of the photometer = ;;^ = loo) the intensity 

 of the illumination at the field of the photometer 

 (= £-,*) will be expressed by the fraction yor- = totooJ 

 and the relative intensity of any other light which is 



* See Page 5. 



