NEWTON. 



215 



reihtion to a hypothesis of his own that light is 

 simply the effect of vibrations excited and propagated 

 in an elastic medium and concluded by allowing 

 whatever appeared reconcilable with his own hypo- 

 thesis, and by advising Newton not to seek any other 

 explanation of the facts. Newton in reply (Phil. 

 Trans., vii.), after exposing some errors of Hooke, 

 adduces new experiments confirming his former 

 results, and refutes the objections to the production 

 of whiteness by the mixture of all the rays. To 

 several other attacks (particularly one by Huygens), 

 which appeared in the Philosophical Transactions, 

 and which were conducted on similar principles, he 

 was obliged to reply. In vain did he declare that 

 he neither advanced nor admitted any hypothesis 

 whatever, and that his sole object was to establish 

 and connect facts by means of the laws of nature. 

 This severe and abstract method of reasoning was 

 little understood, and it is hardly conceivable into 

 what minuteness of detail he was obliged to enter. 

 So much was he disgusted with these difficulties, that 

 he gave up his intention of printing his lectures on 

 Optics with his treatise on Series. Before quitting 

 the lists, however, he addressed another paper (1675) 

 to the royal society, completing the account of his 

 results, and of his views on the nature of light. This 

 treatise, united with his first paper on the analysis or 

 light, afterwards served as the base of the great work, 

 Treatise on Optics (1704), in which, however, the 

 experimental investigation of the phenomena is more 

 extensive, and more strictly separated from all hy- 

 pothesis. The new experiments with which it was 

 enriched, relate principally to the colours observed in 

 thick plates of all bodies, when they are presented, in 

 a proper manner, to the incident ray. Newton re 

 duced them to the same laws as those of the pheno- 

 mena in thin plates ; and then, considering these 

 laws as established facts, equally certain with the 

 particular experiments from which they are deduced 

 yet far more universal, he unites them all in one 

 general property of light, each peculiarity of which 

 is characterized with such exactness as to make the 

 general property a pure expression for all the observed 

 laws. The essence of this property is, that each 

 particle of light, from the instant when it quits the 

 radiating body whence it emanates, is subject, peri- 

 odically, and at equidistant intervals, to a continual 

 alternation of dispositions, to be reflected from, or 

 transmitted through, the surfaces of the diaphanous 

 bodies which it meets with ; so that, for instance, if 

 such a surface presents itself to the luminous particle 

 during one of the alternations, when the tendency to 

 reflection, which Newton called the " fit of easy re- 

 flection," is in force, this tendency makes it yield 

 more easily to the reflecting power of the surface, 

 while, on the other hand, it yields with more diffi- 

 culty when it is in the contrary phase, which he 

 termed the " fit of easy transmission." (See Light 

 and Optics.) In his paper of 1675, after excusing 

 himself for proposing a conjecture as to the nature of 

 light, and declaring that it had no connexion with the 

 facts which he had discovered, he goes on to give one 

 "Inch he should be inclined to consider most pro- 

 bable, if he were obliged to adopt any. He then 

 admits the existence of an imperceptible fluid (which 

 he calls aether], extending everywhere in space, and 

 penetrating all bodies with different degrees of den- 

 sity. This fluid he considers as highly elastic, and. 

 consequently, pressing against itself and the material 

 parts of other bodies, with an energy proportional to 

 its actual density. If this asther be disturbed or 

 agitated, in any one point, by any cause which pro- 

 duces a vibratory motion, this motion must, transmit 

 itself, by undulations, through all the rest of the 

 medium ; and if these undulations encounter, in their 



passage, the material particles forming the substance 

 of any body, they will agitate them with considerable 

 force. Now, light, he admits, consists of a peculiar 

 substance, different from the Esther, but composed of 

 heterogeneous particles, which, darting in all direc- 

 tions from luminous bodies, with great velocity, agi- 

 tate the asther in their passage, and excite undula- 

 tions. He does not attempt to determine the essence 

 of these particles. 



From this time till 1679, Newton communicated 

 nothing to the royal society, and in this interval ap- 

 pears to have been occupied with astronomical ob- 

 servations. In that year he had occasion to write to 

 Hooke about a system of physical astronomy, on 

 which the royal society had asked his opinion. In 

 his letter he proposed, as a matter deserving attention, 

 to verify the motion of the earth by direct experi- 

 ment, viz. by letting bodies fall from a considerable 

 height, and observing if they follow exactly in a verti- 

 cal direction ; for if the earth turns, since the rotary 

 velocity at the point of departure must be greater 

 than at the foot of the vertical, they will be found 

 to deviate from this line towards the east, instead of 

 following it exactly, as they would do if the earth did 

 not move. Hooke replied, that wherever the direc- 

 tion of gravity is oblique to the axis of the earth, 

 bodies in falling change parallels, and approach the 

 equator. This led Newton to consider whether the 

 elliptical motion of the planets could result from a 

 force varying inversely as the square of the distance, 

 and, if so, under what circumstances such a result 

 would ensue. In proposing his experiment to the 

 society, he had considered the motion of the heavy 

 body as determined by a force of constant intensity, 

 and had concluded the trajectory to be a spiral, 

 doubtless because he imagined the body to fall in a 

 resisting medium, such as the air. Hooke replied, 

 that it should not be a spiral, but that in a vacuum it 

 would be an eccentric ellipse, which, in a resisting 

 medium, would change into an eccentric ovoidal 

 curve ; and he represented the eccentric ellipse as the 

 consequence of a force inversely proportional to the 

 squares of the distances from the earth's centre. 

 Newton, having examined this result by mathemati- 

 cal calculations, found that an attractive force, ema- 

 nating from a centre, and acting inversely as the 

 squares of the distances, would produce motions ex- 

 actly resembling the planetary motions, both in regard 

 to the form of the orbit and the velocity of the body 

 j at each point. This was the secret of the system 

 of the world ; but it still remained to account for 

 the discordance of the moon's motion, which had 

 before (1665) embarrassed Newton. But, in 1682, 

 having learned the results of the new measurement 

 of a degree by Picard, he resumed his former calcula- 

 tions from these data. Finding, as he advanced, the 

 manifest tendency of these numbers to produce the 

 long-desired results, he became so much agitated as 

 to be unable to go on with his calculation, and re- 

 quested one of his friends to finish it. Two years 

 were spent in penetrating the consequences of this 

 discovery, and preparing his immortal work, Philoso- 

 phic Naturalis Principia Mathematica, during which 

 time he lived only to calculate. He would sometimes 

 rise, and, suddenly arrested by some new conception, 

 would sit on his bedside for hours together, and would 

 forget his meals, unless reminded of the necessity of 

 taking nourishment. It was not till 1686, that he 

 finally concluded to present his work to the society, 

 at the expense of which it was printed in 1687. Not 

 more than two or three of his contemporaries were 

 capable of understanding it, and more than fifty years 

 elapsed before the great physical truth which it con- 

 tained was thoroughly understood by the generality o 

 scientific men. 



