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LXVIII. On a Theory proposed by Fresnel, and on a mode of 

 measuring the average size of very fine Particles. By Ogden 

 N. Rood, Professor of Physics in Columbia College*. 



IF the light from a candle-flame be received on a ground glass 

 surface so obliquely that the incident ray makes only a very 

 small angle with the glass surface, the light will be copiously 

 reflected, and a bright uncoloured image of the flame will be seen 

 by reflexion. As the angle made by the incident ray is increased, 

 the reflected image becomes first yellow, then red, and finally 

 disappears altogether. 



Fresnel has attempted to account f for this fact on the ground 

 that the more refrangible rays, having shorter wave-lengths, are 

 caused to interfere by a difference of path, which is still too small 

 to effect complete interference in the case of the longer waves of 

 red light — the difference in path depending on the depth of the 

 minute scratches on the surface of the glass, and on the angle 

 which the ray makes with this surface. 



As it is not difficult to measure approximately the angle at 

 which the red ray ceases to be reflected, it would be easy to put 

 this theory to the test of experiment if the average depth of the 

 scratches on the ground surface were known. 



The impossibility of obtaining such measurements has hitherto 

 prevented this theory of the action of finely roughened surfaces 

 on light from being either confirmed or overthrown. 



Some time ago, while experimenting on a plane polished sur- 

 face of glass which had been smoked with lampblack to complete 

 opacity, I was surprised to find that the lampblack surface at 

 a great obliquity reflected all the rays of light with much bril- 

 liancy, so that it resembled in appearance a polished surface of 

 metal or glass. With less degrees of obliquity the reflected light 

 was yellow, red, and finally disappeared altogether. 



The. lampblack surface in this experiment was obtained from 

 burning paraffine, and it was found that the red ray ceased to 

 be reflected at an angle of 18°, reckoning from the glass surface. 

 The source of light was a small gas-flame; and the experiments 

 were made in a darkened room at night, the glass plate with its 

 lampblack surface being attached to the axis of a graduated cir- 

 cle, the lampblack having been removed from the upper half of 

 the plate so as to allow the proper adjustments to be made with 

 the aid of the naked glass surface. I then attempted to mea- 

 sure with the microscope the average size of the smaller and 

 more numerous particles of lampblack ; the result obtained was 



* From Silliman's American Journal for January 1867. 

 t Poggendorff's Annalen, vol. xii. p. 210. 



