134 Astronomical and Nautical Collections, 



this case the internal fringes, which disappear, upon the in- 

 terception of either of the pencils, are much finer than the 

 others, which depend on each slit separately ; and it is to- 

 wards the middle of the space in which these coarser stripes 

 mix with each other, that the finer lines are generated. 



It is always supposed that the whole of the light, em- 

 ployed in these experiments, is derived from the same lu- 

 minous point ; for if it were otherwise, if the two pencils 

 did not emanate from the same source, the efi*ects of which 

 we have spoken would no longer take place. The reason of 

 this will appear hereafter : for the present, it will be best to 

 confine ourselves to those facts which demonstrate, with the 

 strongest evidence, that, in certain cases, the rays of light 

 exert a sensible influence on each other. 



In order to complete what belongs to this part of the 

 subject, it is necessary to describe another experiment which 

 exhibits this influence in a very neat manner, and has the 

 advantage of separating it from the phenomena of diff*rac- 

 tion properly so called. It consists in employing two mir- 

 rors, very slightly inclined to each other, for the reflection 

 of rays proceeding from the same luminous point; and it 

 will be best to premise an account of some more general im- 

 provements which may be made in the means of observa- 

 tion. 



Instead of employing for a luminous point a small hole made 

 in a piece of tinfoil, or of card placed in a window-shutter, 

 it is much more convenient to insert in it a convex glass lens 

 of a very short focus, on which the sun's rays are thrown 

 horizontally by a mirror outside of the window. It is well 

 known that the efi*ect of such a lens is to unite into a point, 

 called its focus, all such rays as fall parallel to each other 

 on its surface ; and that this focus is situated in the ray which 

 passes through the centre of the lens, and is so much the 

 nearer to it as the lens is more convex. In order to have a 

 more distinct idea of the experiment, we may suppose this 

 focal distance to be a centimetre, or about ^*o of an inch. 

 If the sun, like the fixed stars, exhibited to us no appre- 

 ciable angular diameter, all the rays after refraction would be 

 united into a single focus ; but the sun actually subtends an 



