of the Light emitted by Ltiminous Bodies. 19 



compared with it, according to the directions before 

 given, may be found by the following proportion : Call- 

 ing this light B, and putting y = its intensity at its 

 source, and n = its distance from the field of the pho- 

 tometer, expressed in English inches, as it is J ^ 5 (^^ 

 was before shown), or, instead of £^, writing its value 

 = TO 0' it will be J = xoooo' and consequently jj/ is 

 to I as n is to 10,000; or the intensity of the light B 

 at its source is to the intensity of the standard light A 

 at its source as the square of the distance of the light B 

 from the middle of the field of the instrument, expressed 

 in inches, is to 10,000; and hence it is y =^ lo-f-oo"- 



I have been the more particular in this account 

 of the instruments employed in these inquiries, the 

 manner in which the experiments were conducted, 

 and the principles upon which the conclusions drawn 

 from them are founded, not only because, the subject 

 being new, the most particular information upon all 

 these points is absolutely necessary, to enable others to 

 judge with certainty of the matter submitted to their 

 examination, but also because I was very desirous 

 of affording every information and assistance in my 

 power to those who may be disposed to prosecute 

 these curious and entertaining researches. 



Hoping that this apology may be thought sufficient 

 to excuse the prolixity of these descriptions, I shall 

 now proceed to give a short account of such experi- 

 ments as I have hitherto found leisure to make with 

 this apparatus. 



My first attempts were to determine how far it 

 might be possible to ascertain, by direct experiments, 

 the certainty of the assumed law of the diminution of 

 the intensity of the light emitted by luminous bodies ; 



