140 Astronomical and Nautical Collections, 



The direction of these stripes is always perpendicular to a 

 right line, supposed to join the two images, at least in the 

 «pace enlightened by the rays that are regularly reflected, 

 \vhatever the direction of that line may be with regard to 

 the line of contact of the mirrors ; which is sufficient to show 

 that they do not depend on any influence exerted by the 

 edges of the mirrors on the light that passes near them. Be- 

 sides, by altering the angle which the mirrors form with 

 each other, we may separate the images so far from each 

 other, as to cause the rays which form them to be reflected 

 at too great a distance from the edges in contact, to allow us 

 to suppose that any sensible influence can be exerted on the 

 light by these edges. 



The middle stripe is always brilliant, like that of the 

 fringes which are seen in the shadow of a narrow body, or 

 those which are obtained from a plate furnished with two 

 fine parallel slits very near each other. This bright stripe 

 is situated between two dark stripes of the deepest black, 

 when we employ homogeneous light : each of these is fol- 

 lowed by a bright stripe, and these again are succeeded by 

 dark stripes, and so forth. The dark stripes are still of a 

 dark black in the fringes of the second and third orders ; 

 but, in proportion as we go further from the middle, the 

 tint is less decidedly black, because the light employed is 

 never perfectly homogeneous. 



It is sufficient to compare the three first pairs of dark 

 stripes with the light affi^rded by a single mirror, to con- 

 vince ourselves that they are much less enlightened, and that 

 in the places which they occupy, the addition of the rays of 

 one of the mirrors to those of the other, instead of increas- 

 ing the light, actually produces darkness. It is easy to make 

 this comparison by looking successively at the black stripes, 

 and at the enlightened parts situated to the right and left of 

 the space which is doubly illuminated, and in which the 

 fringes are found. If we were apprehensive that the con- 

 trast with the bright stripes in the neighbourhood might 

 occasion some fallacy in this respect, it would be sufficient 

 to place the thread of the micrometer successively in one of 

 the black stripes near the middle, and in the space enlight- 



