338 Dr. G. J. Stoney on Microscopic Vision. 



our choice as regards these optional matters, the plane waves 

 emanating from the whole field into which the light emitted 

 by the pointy is to be resolved under our theorem, become 

 as definite and unique as do the coefficients of a Fourier's 

 series when once we have decided on the direction of the fine 

 mm! and have selected a value for a. This, however, still 

 leaves a considerable latitude under our theorem, as to what 

 the undulations of plane waves shall be, since the objective 

 field may be variously chosen, and the only conditions which 

 limit the positions to be selected for the replicas are that they 

 and the original objective field be equally spaced relatively to 

 the objective plane, and that the nearest of the replicas shall 

 lie far enough outside the objective field to ensure that 

 whether sources of light exist in them or not shall not sensibly 

 interfere with what is seen by the observer. They are to 

 him stars below his horizon, whose positions or even existence 

 in no perceptible degree affect the distinctness with which he 

 sees the stars that are above his horizon. 



8. Principles of Reversed. — A further insight into what it is 

 that occurs may be gained by a simple expedient. Picture a 

 portion of the objective plane, of limited but large size — large 

 enough to have the original objective field near its middle, 

 and a great many of its replicas disposed round it. If all of 

 these emit light that is exactly similarly circumstanced, then, 

 as already explained, it appears that they, acting together, 

 will produce undulations of very nearly uniform plane waves 

 which will become more and more disentangled from one 

 another the farther out they go. It is in fact when thus 

 disentangled that their consisting of almost quite plane waves 

 becomes most obvious. The approximation to accurately 

 uniform plane waves can, of course, be carried as far as we 

 please by increasing the number of replicas engaged in 

 emitting the light. 



Let now all the setherial motions be suddenly reversed, and 

 let at the same time the objective field and its replicas be got 

 out of the way. The distant undulations winch were before 

 advancing outwards will now travel inwards without ceasing 

 to be uniform plane waves, and will by simple geometrical 

 superposition, according as they overlap one another, repro- 

 duce at each step of their inward journey exactly the same 

 disturbed state of the aether as had prevailed at the same 

 stations on the outward journey, except that the directions of 

 all motions are reversed. Hence plane waves converging in- 

 wards would by their superposition produce precisely the same 

 disturbance in the aether, except only with reversed motions, 

 as that which on the outward journey prevailed close in front 



