542 



EXPERIMENTS AND INQUIRIES 



Why, of the same kind of rays, in every cir- 

 cumstance precisely similar, some should al- 

 ways be reflected, and others transmitted, 

 appears in this system to be wholly inexpli- 

 cable. 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. Whe- 

 tlier 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 refractive 

 power of a fluid undergoes any change by 

 electricity. The uniformity of the motion 

 of light in the same medium, which is a dif- 

 ficulty in the Newtonian theory, favours the 

 admission of the Huygenian ; as all impres- 

 sions are known to be transmitted through 

 an elastic fluid with the same velocity. It 

 has been already shown, that sound, in all 

 probability, has very little tendency to di- 

 verge : 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 tlie experiments of Newton and of Mr. 

 Jordan, was not partly occasioned by the 

 tendency of light to diverge ; nor indeed has 

 any other probable cause been yet assigned 

 for its appearance. Eulcr's hypothesis, of 



the transmission of light by an agitation of 

 the particles of the refracting media them- 

 selves, is liable to strong objections ; accord- 

 ing to this supposition, 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 at- 

 traction, a greater or less quantity of the lu- 

 minous ether, so as to make its density 

 greater than that which it possesses in a va- 

 cuum, without increasing its elasticity ; and 

 that light is a propagation of an impidse 

 communicated to this ether by luminous bo- 

 dies : whether this impulse is produced by a 

 partial emanation of the ether, or by vibra- 

 tions of the particles of the body, and whether 

 these vibrations, constituting white light, are, 

 as Euler supposed, of various and irregular 

 magnitudes, or whetjier they are uniform, 

 and comparatively large, remains to be 

 hereafter determined; although the opinion of 

 Euler respecting them seems to be almost the 

 only one which is consistent with the New- 

 tonian discoveries. Now, as the direction of 

 an impulse, transmitted through a fluid, de- 

 pends 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 small elas- 

 tic body strikes against a larger one, it is 

 well known that the smaller is reflected more 

 or less powerfully, according to the diflerence 

 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 fre- 

 quently an echo when a sound strikes against 

 a cloud. A greater body, striking a smaller 



