84 Count Rumford’s Account of a Method of 
the field of the photometer =_m= 100 ;) the intensity of the 
illumination at the field of the photometer (= £*) will be 
expressed by the fraction ===r = Toko 5 and l ^ e r e l ative in- 
tensity of any other light which is compared with it, according 
to the directions before given, may be found by the following 
proportion : calling this light B, 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 ■, as was 
shewn in my former letter, or instead of writing its value 
= 7 -^ it will be = Tofeo’ and consequently y is to 1 as 
n is to 10000 ; 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 10000 ; 
and hence it is y = -—55. 
Or, if the light of the sun or that of the moon be compared 
with the light of a given lamp or candle C, the result of such 
comparison may best be expressed in words, by saying, that 
the light of the celestial luminary in question, at the surface of 
the earth , or which is the same thing, at the field of the pho- 
tometer, is equal to the light of the given lamp or candle, at the 
distance found by the experiment ; or putting a = the intensity of 
the light of this lamp Cat its source, and = its distance in inches 
from the field, when the shadows corresponding to this light, 
and that corresponding to the celestial luminary in question, 
are found to be of equal densities ; and putting z ~ the inten- 
sity of the rays of the luminary at the surface of the earth, 
the result of the experiment may be expressed thus, %=. — v 
* See my former letter. 
