128 Astronomical and Nautical Collections. 



But there is another much more powerful reason which 

 prevents our perceiving the effects of the mutual influence of 

 the systems of waves when the difference of their routes is 

 considerable; which is the impossibility of rendering the 

 light sufficiently homogeneous : for the most simple light 

 that we can obtain consists still of an mfinity of heteroge- 

 neous rays, which have not exactly the same length of undu- 

 lation ; and however slight the difference may be, when it is 

 repeated a great number of times, it produces of necessity, 

 as we have already seen, an opposition between the modes of 

 interference of the various rays, which then compensates for 

 the weakening of some by the strengthening of others ; 

 [while the shades of colour are not sufficiently distinct to 

 allow the eye to remark the difference.] This is without 

 doubt the principal reason why the effects of the mutual 

 interference of the rays of light become insensible when the 

 difference of the routes is very considerable, so as to amount 

 to 50 or 60 times the length of an undulation. 



It has already been laid down as one of the conditions 

 necessary for the appearance of the phenomena of interfer- 

 ence, that the rays which are combined should have issued 

 at first from a common source : and it is easy to account for 

 the necessity of this condition by the theory which has now 

 been explained. 



Every system of waves, whicli meets another, always exer- 

 cises on it the same influence when their relative positions 

 are the same, whether it originates from the same source or 

 from different sources ; for it is clear that the reasons, by 

 which their mutual influence has been explained, would be 

 equally applicable to either case. But it is not sufficient 

 that this influence should exist, in order that it may become 

 sensible to our eyes : and for this purpose the effect must 

 have a certain degree of permanence. Now this cannot 

 happen when the two systems of waves which interfere are 

 derived from separate sources. For it is obvious that the 

 particles of luminous bodies, of which the vibrations agitate 

 the ether, and produce light, must be liable to very frequent 

 disturbances in their oscillations, in consequence of the rapid 

 changes which are taking place around them, which may 



