measuring the comparative Intensities of Light. 85 
or the real value of a being determined by a particular expe- 
riment, made expressly for that purpose with the standard 
lamp, that value may be written instead of it. When the 
standard lamp itself is made use of, instead of the lamp C, then 
the value of a will be 1 . 
I have been the more particular in this account of the instru- 
ments employed in these inquiries, the manner in which the ex- 
periments 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 exami- 
nation, 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 very curious and entertaining re- 
searches. 
Hoping that this apology may be thought sufficient to ex- 
cuse the prolixity of these descriptions, I shall now proceed to 
give a short account of such experiments 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 ; namely, that the intensity of the 
light is every where as the squares of the distances from the 
luminous body inversely. These experiments appeared to me 
the more necessary, as it is quite evident that this law can only 
hold good when the light is propagated in perfectly trans- 
parent or unresisting spaces, or where, suffering no diminu- 
tion whatever from the medium, its intensity is weakened 
