328 Mr. H. F. Talbot's Experiments on Light. 



least no instrument that has been proposed has met with ge- 

 neral approval and adoption. I am persuaded, nevertheless, 

 that light is capable of accurate measurement, and in various 

 ways; and that the difficulties which stand in the way of ob- 

 taining a convenient and accurate instrument for photometricai 

 purposes will ultimately be overcome. 



To begin with the most simple considerations which may 

 serve to guide us in reasoning on this subject, I will select 

 the experiment well known to every one of whirling round a 

 glowing coal, which from its rapid motion conveys to the eye 

 the impression of a continuous fiery circle. The question de- 

 serves consideration whether the eye receives from this cir- 

 cular ring exactly the same quantity of light which it received 

 from the much smaller surface of the coal at rest. There can 

 be no doubt, as it seems, that such must be the case ; for if 

 the luminous circle sent more rays to the eye, it would like- 

 wise send more in every other direction, and thus the apart- 

 ment would become more illuminated than before, which is 

 not the fact. If, then, the total quantity of light remains the 

 same, it follows that its apparent intensity must have dimi- 

 nished exactly in the same proportion as its apparent area has 

 been enlarged. For the sake of more accuracy if we confine 

 our reasoning to a very small portion of the luminous body, 

 the enlarged area which it seems to occupy is evidently pro- 

 portional to the circumference of the circle it describes ; there- 

 fore the preceding argument alleges that the intensity of light 

 diminishes as the circumference increases. But in the same pro- 

 portion in which the circumference increases, the time during 

 which the coal is found in any particular point of the circle 

 diminishes. 



The rapidity of the rotation does not affect the argument. 

 For instance, if the rotation is in a vertical plane, the time 

 during which the coal occupies the summit of the circle, com- 

 pared with the time of one whole revolution, necessarily di- 

 minishes when the circle grows larger; for the time of its 

 being so situated bears the same proportion to the whole 

 time, that the diameter of the moving body (which is con- 

 stant) bears to the whole circumference through which it 

 moves. Since, then, these two things— the intensityof light 

 and the time of the body's remaining in any given part of the 

 circle — are each inversely proportional to the circumference of 

 the circle it describes, it follows that they must be directly 

 proportional to each other ; that is to say, that a regularly in- 

 termittent luminary whose observations are too frequent and 

 too transitory for the eye to perceive, loses so much of its ap- 

 parent brightness from this cause, as is indicated by the pro- 



