KINETIC OK MECHANICAL VIEW OF NATURE. 15 



suggesting tluiL the rays of light were possessed of tits of 

 eusy transmission and reflexion, i.e., of regular periodic 

 changes which could be measured and numbered. To 

 this amplification of the simple geometrical emission 

 theory Newton was driven by his own immortal researches, 

 which revealed the wonderful regularly arranged colours 

 of thin plates known as Newton's rings. In reading, 

 after the lapse of nearly two centuries, the reflections of 

 Newton on the nature of light, reflections which he never 9. 

 gathered up mto a compact and exhaustive treatise, as he minpests 

 did the theory of gravitation,^ we recognise that he had t'-eory. 

 clearly before his mind the two fundamental phenomena 

 peculiar to light, namely, its property of travelling in 

 straight lines, and its periodicity, as revealed by certain 

 delicate experiments of his own. Which of the two 

 theories should in the end prevail depended on the more 

 intimate knowledge — to be gained by experiment and 

 calculation — of the two kinds of motion involved ; of 

 rectilinear motion of particles under the influence of 

 contending forces, and of the more complicated periodic 

 motion pecuhar to waves, tremors, or oscillations. The 

 first kind of motion, being more easily studied and 

 also more nearly related to other prevailing studies, 

 received earlier attention ; the second — especially so 



^ It is now sufficiently known 

 and recognised that Newton, both 

 in the theory of gravitation and 

 that of Hglit, did not pro[io8e to do 

 more than give a preliminary formu- 

 lation which was applicable as a 

 ba-sis for e.\perimentand calculation. 

 His further speculations are con- 

 tained mostly in the well - known 

 'yu<?riea' to the 'Opticks,' which 



were extended in later editions, and 

 among which, " to show that " he 

 " did not take gravity for an esseu- 

 tial property of bodies," he adiled 

 one (juestion concerning its cause, 

 choosing to projiose it by way of a 

 question, because " he was not yet 

 satisfied about it for want of experi- 

 ments" (Advertisement to second 

 edition, 1717). 



