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. the Method of performing the simple Experiment 
of Interferences nxith two Mh'rors slightly inclined, so as to 
afford an experimentum crucis as to the nature of Eight, 
By R. Potter, Esq,^ 
Tj^RESNEUS 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 7iea7'ly 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 necessar^^ 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. 
