450 Astronomical and Nautical Collections, 



if the external fringes were derived from the meeting of the 

 direct rays with rays reflected from the edge of the opaque 

 body ; for then the points of maximum or minimum of illu- 

 mination, at difi^erent distances from the body, would be 

 situated in hyperbolas, having for their foci the luminous 

 point and the edge of the body, as may easily be inferred 

 from the very simple law of the mutual influence of the 

 rays. It is not, indeed, by the combination of these rays 

 only, that the external fringes are produced, for an infinity 

 of other rays, inflected near the body, concur in producing 

 them ; but the trajectories of these are curves of the same 

 nature, and these dark and bright shades are always derived 

 from the mutual action of the rays, without which it would 

 be impossible to understand their curvilinear progress. — 

 Whatever system, therefore, we adopt, it becomes necessary 

 to admit that the rays of light exert this mutual influence ; 

 which, indeed, is so fully established by the experiments 

 already related, that we may now consider it as one of the 

 best established principles in optics. 



It is very difficult to conceive the existence of a similar 

 phenomenon upon the system of emission, which does not 

 allow us to suppose any connexion between the motions 

 of the several particles of light, without destroying the 

 fundamental hypothesis of the system. We should, at any 

 rate, be obliged to admit, that the mutual action of the 

 rays in such cases is only apparent, and has no real ex- 

 istence; or, in other words, that the phenomenon takes 

 place within the eye only, where the successive impulses 

 of the luminous molecules, on the optic nerve, might be 

 imagined to augment or to diminish the magnitude of the 

 vibrations already commencing, accordingly as they might 

 tend to favour or to check the elementary motions con- 

 stituting these vibrations: in the same manner as a ringer, 

 when he is raising a bell into its swing, is obliged not only 

 to multiply his efforts on the rope, but to leave between 

 them a proper and regular period, depending on the capa- 

 bility of the bell to perform its vibrations, so that each new 

 impulse may tend to co-opera,te with the motion already 

 communicated to it. 



