6l4 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



But a still more insuperable difficulty seems to occur, in the partial reflection from 

 every refracting surface. Why, of the same kind of rays, in every circumstance 

 precisely similar, some should always be reflected, and others transmitted, appears 

 in this system to be wholly inexplicable. That a medium resembling, in many 

 properties, that which has been denominated ether, does really exist, is undeniably 

 proved by the phenomena of electricity ; and the arguments against the existence 

 of such an ether throughout the universe have been pretty sufficiently answered 

 by Euler. The rapid transmission of the electrical shock shows that the electric 

 medium is possessed of an elasticity as great as is necessary to be supposed for the 

 propagation of light. Whether the electric ether is to be considered as the same 

 with the luminous ether, if such a fluid exists, may perhaps at some future time be 

 discovered by experiment; hitherto I have not been able to observe that the refrac- 

 tive power of a fluid undergoes any change by electricity. The uniformity of the 

 motion of light in the same medium, which is a difficulty in the Newtonian theory, 

 favours the admission of the Huygenian; as all impressions are known to be trans- 

 mitted through an elastic fluid with the same velocity. It has been already shown 

 that sound, in all probability, has very little tendency to diverge: in a medium so 

 highly elastic as the luminous ether must be supposed to be, the tendency to diverge 

 may be considered as infinitely small; and the grand objection to the system of 

 vibration will be removed. It is not absolutely certain, that the white line visible 

 in all directions on the edge of a knife, in the experiments of Newton and of Mr. 

 Jordan, was not partly occasioned by the tendency of light to diverge. Euler's 

 hypothesis of the transmission of light, by an agitation of the particles of the 

 refracting media themselves, is liable to strong objections; according to this suppo- 

 sition, the refraction of the rays of light, on entering the atmosphere from the 

 pure ether which he describes, ought to be a million times greater than it is. For 

 explaining the phenomena of partial and total reflection, refraction, and inflection, 

 nothing more is necessary than to suppose all refracting media to retain, by their 

 attraction, a greater or less quantity of the luminous ether, so as to make its den- 

 sity greater than that which it possesses in a vacuum, without increasing its elasti- 

 city; and that light is a propagation of an impulse communicated to this ether by 

 luminous bodies: whether this impulse is produced by a partial emanation of the 

 ether, or by vibrations of the particles of the body, and whether these vibrations 

 are, as Euler supposed, of various and irregular magnitudes, or whether they are 

 uniform, and comparatively large, remains to be hereafter determined. Now, as 

 the direction of an impulse transmitted through a fluid, depends on that of the 

 particles in synchronous motion, to which it is always perpendicular, whatever 

 alters the direction of the pulse, will inflect the ray of light. If a smaller elastic 

 body strike against a larger one, it is well-known that the smaller is reflected more 

 or less powerfully, according to the difference of their magnitudes: thus, there is. 

 always a reflection when the rays of light pass from a rarer to a denser stratum of 

 ether; and frequently an echo when a sound strikes against a cloud. A greater 



