380 Mr. Potter on FresneVs Experiment of Interferences 



with which they are struck will depend not so much on the 

 velocity as the momentum of the large floating islands. The 

 same berg is often carried away by a change of the wind and 

 then driven back again upon the same bank, or in other cases 

 it is made to rise and fall by the waves of the ocean, and may 

 thus alternately strike the bottom with its whole weight, and 

 then be lifted up again until it has deranged the superficial 

 beds over a wide area. On these beds new and undisturbed 

 strata may be afterwards thrown down. In other cases, when 

 banks of mud and sand forming the top of a shoal have been 

 made to assume various shapes by the lateral pressure of ice- 

 bergs, the bed of the sea may subside, and then the disturbed 

 beds may be overspread by horizontal strata, which may never 

 afterwards be deranged by similar mechanical violence. 



LVIII. On the Method of performing the simple Experiment 

 of Interferences isoith two Mirrors slightly inclined, so as to 

 afford an experimentum crucis as to the nature of Light, 

 By R. Potter, Esq.^ 



Tj^RESNEL'S genius devised the experiment which is the 

 most direct test of the reality of the interference of light, 

 and which proves that property in the most unequivocal man- 

 ner. This experiment is performed by causing the light di- 

 verging from a luminous point to be reflected by two plane 

 mirrors, placed side by side, whose surfaces are nearly in the 

 same plane, but which contain an angle a little less than 180°, 

 and then examining the light by means of an eye-lens. Each 

 mirror gives an image of the luminous point, and we have the 

 reflected light proceeding as if it diverged from these two images 

 and also from its having originally constituted only one pencil, 

 the two reflected pencils are in the same state, so that they 

 interfere where they cross each other's direction, producing 

 in ordinary light coloured bands parallel to the line of inter- 

 section of the planes of the two mirrors, with dark intervals 

 between them. These bands are seen in the air in the focus 

 of the eye-lens when looking towards the images of the lu- 

 minous point. 



Without examining the experiment more minutely than 

 just to ascertain that both the pencils are necessary to the 

 production of the bands, it must be admitted that it is con- 

 clusive in establishing the theory of interferences. 



The theory of interferences was brought forward by Dr. 

 Young as a consequence of the undulatory or wave theory of 



* Communicated by the Author. 



