Astronomical and Nautical Collections* 445 



The explanation, which has been given, of the small num- 

 ber of fringes produced by white light, and of the limited 

 number which are perceptible in the most homogeneous light 

 that can be procured, is sufficient to inform us, why, k 

 many cases of the crossing of the rays derived from com- 

 mon sources in directions nearly parallel, there is no pro- 

 duction of fringes whatever : since the difference of the 

 paths described is too considerable, and is too great a 

 multiple of d, at all the points of the space enlightened by 

 the two pencils united; so that the middle of the fringes, 

 and the stripes near enough to it to be perceptible, corre- 

 spond to points without the common field of the double 

 illumination. 



This is the reason why it is so important, in the experi- 

 ment with the mirrors, that they should not project one be- 

 yond the other ; for, on account of the extreme minuteness 

 of the quantity rf, which is only about one fifty thousandth 

 of an inch for the yellow rays, the slightest inequality of 

 projection, producing a difference in the length of the path 

 equal to twice its own magnitude, may throw the group of 

 visible fringes beyond the common field of the two mirrors. 

 It may also be observed, in such experiments, that besides 

 the rays regularly reflected by the mirror, there are always 

 softie which are inflected in the neighbourhood of their ter- 

 minations, and which enlarge in this manner the space of the 

 double illumination. The rays regularly reflected by one of 

 the mirrors, interfering with the rays inflected near the 

 circumference of the other, may also produce fringes, when 

 the difference of the paths described is very small; but 

 these fringes are distinguished in general from those which 

 depend on the interference of the rays regularly reflected, 

 by their curvilinear form, and by their direction, which is not 

 perpendicular to that of the line joining the two images of 

 the luminous point. 



The reasoning, which has been employed for explaining 

 the colours of the fringes produced by the mutual influence 

 of two white pencils, may be applied to all the phenomena 

 of diffraction in white light. These effects depend always 

 on the difference of the breadths of the dark and bright 



