Astronomical and Nautical Collections* 433 



When the two systems of waves which interfere are paral- 

 lel, the interval which separates their corresponding points 

 niust remain the same for a great portion of the surface of 

 the waveSt that is to say, in other words, the fringes will 

 become almost infinite in breadth, so that <i very considerable 

 displacement of the centre of undulation will cause very 

 little difference in the agreement or disagreement of their 

 vibrations. And in this case it is no longer necessary to 

 employ so small an objecttifl ,oj:d^r,,to^p?^Wye,t}ie ^ffect^ of 

 their mutual influence, it hm^Imh tftfv»>(rf Mf.in^ .^rU in ^iv, ).\ 



If the coloured rings, which are produced by the interfer- 

 ence of two systems of undulations nearly parallel, exhibit, 

 like the fringes, and often within a very short distance, alter- 

 nations of dark and bright stripes; this circumstance depends 

 entirely on the want of uniformity in the thickness of the 

 plate of air interposed between the glasses, which causes a 

 variation of the difference of the routes of the rays reflected 

 at the first and at the second surface of this plate, of which 

 the mutual interference produces the bright and dark rings. 



We shall readily be able to understand why the luminous 

 rays, although they always exert a certain influence on each 

 other, exhibit it to the eye so seldom, and in cases so much 

 limited, if we consider that it is necessary, for such an exhi- 

 bition, first, that the rays concerned shall have been derived 

 from a common source; secondly, that the difference between 

 their paths shall amount to a limited number of undulations 

 only, even when the light is as homogeneous as possible ; 

 thirdly, that they shall not intersect each other at too great 

 an angle, because the fringes would become so small as to 

 be invisible even with the assistance of a strong magnifier ; 

 and fourthly, unless the riiys are nearly parallel, that the 

 luminous object should be of very small dimensions, and the 

 smaller in proportion as the inchnation of the rays is greater. 

 .Jt has been thought necessary to insist so much at length 

 on the theory of interferences, because of its numerous 

 applications to the calculation of the most interesting of 

 the laws of optical phenomena. These considerations may 

 perhaps appear at first somewhat delicate and difficult of 

 comprehension, notwithstanding the minuteness of the expla-* 



