20 Experiments on the Relative Intensities 



namely, that the intensity of the light is everywhere as 

 the sqiiares of the distances from the luminous body in- 

 versely. 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 

 transparent or unresisting spaces, or where — suffering 

 no diminution whatever from the medium — its inten- 

 sity is diminished merely in consequence of the diver- 

 gency of the rays; and as it is more than probable that 

 air, even in its purest state, is far from being perfectly 

 transparent. 



For greater perspicuity, I shall arrange all my ex- 

 periments and inquiries under general heads, and shall 

 begin by prefixing to those which relate to the subject 

 now under consideration the general title of 



Experiments upon the Resistance of the Air to Light. 



Experiment No. i. 



Two equal wax candles, well, trimmed, and which 

 were found by a previous experiment to burn with 

 exactly the same degree of brightness, were placed 

 together on one side before the photometer, and their 

 united light was counterbalanced by the light of 

 an Argand's lamp, well trimmed, and burning very 

 equally, placed on the other side over against them. 

 The lamp was placed at the distance of lOO inches 

 from the field of the photometer, and it was found that 

 the two burning candles (which were placed as near 

 together as possible, without their flames affecting 

 each other by the currents of air they produced) were 

 just able to counterbalance the light of the lamp at the 



