by Diffusion of Light. 5& 



air. To perceive this it suffices to pour on the smoked surface 

 a drop of alcohol, benzine, sulphide of carbon — in a word, of a 

 liquid which moistens the lampblack ; but in this case the po- 

 larization-phenomena are impaired*. It is better to operate by 

 immersing the smoked surface in a glass trough containing the 

 liquid. This causes, it is true, a little perturbation owing to 

 the circumstance that the liquids (such as alcohol and benzine) 

 dissolve a substance deposited at the same time as the carbon, 

 and become somewhat fluorescent; but this does not prevent 

 the diminution of the intensity of the diffused light (without 

 alteration of the polarization-phenomena) from being distinctly 

 observed. Is it not probable that, if the index of refraction of 

 the liquid were identically the same as that of the lampblack, 

 there would be no light diffused ? 



It seems to me useless to insist further on this difference of 

 interpretation, to which I attach only secondary importance. 

 The question of knowing if the molecules can be regarded as by 

 themselves effective centres of vibrations, or if the action of these 

 molecules is transferred to the aether which surrounds them so 

 as to modify its general density in the interior of a particle, is 

 perhaps subtile and premature. To arrive at its solution, a very 

 interesting part of the labours of M. Lallemand must certainly 

 be taken into consideration ; I mean his photometric researches. 

 He has, in fact, shown that the intensity of the diffused light 

 can be calculated by admitting that the vibration of the diffused 

 ray is the projection of the incident vibration, and supposing that 

 the vibratory motion is propagated with the same energy in all 

 directions. 



On the proper Colours of Bodies, 



We have seen that, at the time of the diffusion by a smoked 

 surface, beside the polarized light there is always a residue of 

 neutral light. To recognize it, we have only to view with an 

 analyzer, under a visual angle of 90°, the surface illuminated by 

 a pencil of light, or to examine it under any angle when the in- 

 cident pencil is previously polarized. 



I have indicated, in my previous Note, the reasons which 

 prevent me from believing that this residue is due to fluores- 

 cence f. I think that it may be explained in part because the 

 lampblack is not composed of pure carbon, partly because this 



* The conditions are then almost the same as when lampblack depo- 

 sited on the hinder surface of a plate of glass is viewed through the glass, 

 which is traversed by the incident pencil — a case of which we have to speak 

 further. 



f Nevertheless this case may present itself: M. Ed. Becquerel has in 

 fact observed a phosphorescence in the lampblack deposited by certain 

 flames ; but a surface smoked at the flame of benzine is not phosphorescent. 



