312 THE SCIENTIFIC PAPERS OF. 



merely suggest whether it might not be the case that, although 

 the dioptric light was the more brilliant in itself, it would never- 

 theless, at a considerable distance, produce the same effect only as 

 the other light for the following reason. If light might be 

 regarded as a vibratory motion of the medium through which it 

 was transmitted, any obstructive matter in the form of haze or 

 smoke must exercise a destructive effect according to the square of 

 the energy of vibration, or intensity of the light. If that were 

 the case, it followed that a brilliant light would in an obstructive 

 medium soon subside into a light of moderate intensity, and 

 thence proceed at a more equal rate of diminution with light pro- 

 ceeding from a less brilliant source but of equal magnitude, the 

 latter being chiefly determined by the extent of light-emitting 

 surface. For instance, one light produced by a candle would be 

 lost sight of, under certain atmospheric conditions, say at a distance 

 of half a mile. But with six lights of the same size placed side 

 by side, a sufficient amount of light would be conveyed to that 

 distance to produce a distinct effect on the eye. In the same way 

 the glare of the gas-lights of London was seen at a distance of 

 twenty or thirty miles, whereas a limited number of more intense 

 lights would be lost to sight at that distance. He therefore 

 thought the quantity of light emitted was of more importance than 

 its intensity in seeking distant effects, a circumstance which had 

 not perhaps been fully considered in estimating the relative value 

 of the electric light, 'as contrasted with the ordinary optical 

 apparatus of extended surface. 



The question had been put, whether the dioptric light was, 

 under all circumstances, better than the catoptric ; and the author 

 of the paper seemed to be much in favour of the dioptric system. 

 Now it appeared to him that, for lights of comparatively short 

 range, the catoptric system could be used with advantage, because 

 the reflecting mirror was the more simple arrangement ; and if its 

 surface could be kept clean, it would reflect the light in a certain 

 definite direction without much loss, provided the parabolic mirror 

 were extended far enough over the light. The principal draw- 

 back appeared to be, that the surface of the parabolic mirror 

 became tarnished ; and in order to prevent that, he would 

 recommend those interested to try pure nickel surfaces, produced 

 by the galvano-plastic process. He had tried them, and he 



