448 HISTORY OF SCIENCE. 



of a number of facts of very diversified character, which had previously 

 been scarcely noticed. In advocating anew the doctrine of undulations. 

 Young is at great pains to invoke, whenever he can, the great authority of 

 Newton, clarum dvenerabilenomen, adducing on each occasion from 

 the writings of his illustrious predecessor all passages which appear to 

 favour the admissibility of his own doctrines. The passages quoted 

 would seem to show that Newton himself had suggested the hypothesis 

 of undulations in a rare medium ; but they are chiefly from the Queries 

 where Newton, in his " Optics," advances a variety of speculations. The 

 citation of these speculations of Newton's certainly caused the doctrines 

 now definitely enunciated by Young to be more attentively considered, 

 particularly by English men of science, among whom any opinion sup- 

 ported by Newton's approval had the highest authority in its favour. 

 Newton's suggestions include the hypothesis of an "ethereal medium " 

 constituted like atmospheric air, but far rarer, subtler, and more strongly 

 elastic ; that by this medium heat may be conveyed through spaces 

 deprived of air ; that by its vibrations light is " put into fits of easy 

 reflection and easy transmission." This medium may pervade all 

 bodies, and by its elastic force expand through all the heavens, where 

 it need not necessarily retard the motions of planets and comets in 

 any sensible degree, for its resistance may be so small as to be incon- 

 siderable. For instance, if the ether (for so Newton calls it) be sup- 

 posed 700,000 times more elastic than- our air (that is, requiring for 

 the same amount of compression a force 700,000 times greater), and 

 be also supposed 700,000 times rarer than air, the resistance it would 

 offer to a body moving through it would be 600,000,000 times less than 

 that offered by water; and so small a resistance would scarcely make 

 any perceptible alteration in the motion of the planets in 10,000 years. 

 The ether is further supposed to be, like air, capable of vibrations or 

 tremors, only the vibrations are far more swift and minute, for while 

 a vibration of the air set up by a man's voice may occupy a space of 

 8 or 12 inches, those of the ether may be reckoned at not more than 

 TooWotii p ar t o f an i nc h i n extent. Such are some of the suggestions 

 thrown out by Newton in the passages whereon Young rests a claim to 

 his predecessor's support. But that Newton's conception was some- 

 thing different from the fundamental postulate of the undulatory hypo- 

 thesis appears from several of the phrases quoted, as, for instance, 

 where he suggests that the vibrations may overtake the rays of light, etc. 

 It would appear that the hypotheses concerning the nature of light 

 in which Newton admitted vibrations, were really much more com- 

 plicated than the undulatory doctrine. An objection which presented 

 itself to Newton's mind against admitting mere undulations as the 

 mode of action, is clearly stated by himself: "Are not all hypotheses 

 erroneous," he asks, "in which light is supposed to consist in pres- 

 sion or motion propagated through a fluid medium ? If it consisted 

 in pression or motion, propagated either in an instant or in time, 



