Astronomical and Nautical Collections. 133 



them, taken separately, produced a uniform illumination 

 only, their joint effect would also be a uniform illumina- 

 tion, unless they exerted some particular influence on each 

 other. 



If we supposed, as it is natural to do, in the system of 

 emission, that the different inflections of the luminous rays, 

 near the bodies, depend on a certain attractive or repul- 

 sive action, exerted by them on the particles of light, it 

 might be imagined that, in this experiment, the action of 

 the remoter edge of the wire might be modified by that of 

 the screen which touched it, so as to be deprived of the 

 property of producing the fringes. This objection might 

 be very much weakened by observing, that the external 

 fringes are not in the slightest degree afiected by the neigh- 

 bourhood of the screen; but Dr. Young removed it alto- 

 gether, by placing the screen at such a distance, that it could 

 not be supposed to have any effect of this kind, and inter- 

 cepting the beam of light either before or after its arrival 

 at the edge of the wire ; for in either case the internal fi'inges 

 were completely annihilated. 



He also showed the mutual influence of two pencils of 

 light, by admitting them through two small holes near each 

 other, and receiving them on a card at some distance from 

 the window : the intermediate space was then occupied by 

 dark and bright lines, which were obviously occasioned by 

 the concourse of the two pencils, since they disappeared as 

 soon as either of the pencils was intercepted, by stopping 

 the hole that transmitted it. 



The fringes are more distinct when, instead of making 

 two holes in the plate of tinfoil, we make in it two parallel 

 slits at the distance of ^V or ^V of an inch ; we then destroy 

 the fringes by stopping either of the slits, although the 

 light spreading from the other remains still perfectly sen- 

 sible. It often happens, when the slits are not very narrow, 

 and when the light is received on a card pretty near to the 

 window, that some fringes are still perceptible when one of the 

 slits is stopped : but ihey are not the fringes in question ; 

 being easily distinguishable from them while the sills remain 

 much narrower than the interval that separates them ; for in 



