476 LECTURE XL. ' 



inadequate, nor were the phenomena at that time sufficiently investigated 

 for a complete solution of the difficulties attending them. 



It was still believed that every refraction actually produces colour, instead 

 of separating the colours already existing in white light; but in the year 1666, 

 Newton first made the important discovery of the actual existence of colours 

 of all kinds in white light, which he showed to be no other than a compound 

 of all possible colours, mixed in certain proportions with each other, and ca- 

 pable of being separated by refraction of any kind. 



About the same time that Newton was making his earliest experiments on 

 refraction, Grimaldi's treatise on light appeared; it contained many inte- 

 resting experiments and ingenious remarks on the effects of diffraction, 

 which is the name that he gave to the spreading of light in every direction 

 upon its admission into a dark chamber, and on the colours which usually ac- 

 company these effects. He had even observed that in some instances the 

 light of one pencil tended to extinguish that of another, but he had not 

 inquired in what cases and according to what laws such an .interference 

 must be expected. 



The discoveries of Newton were not received without some controversy 

 either at home or abroad; the essential points of his theory were, however^ 

 soon established, but Dr. Hooke very warmly opposed the hypothesis which 

 Newton had suggested respecting the nature and propagation of light. On 

 this subject Newton professed himself by no means tenacious ; he was not, 

 however, convinced by Ds. Hooke, and disliked the dispute so much, that he 

 deferred the publication of his treatise on optics till after Hooke's death 

 in 1703. Very soon after his first communication to the Royal Society, in 

 1672, he had sent them a description of his reflecting telescope, which was 

 perhaps the first that had been constructed with success, although Gregory 

 had invented his instrument some years before, and a plan of a similar kind 

 had been suggested by Eskinard as early as l6l5. The principal parts of the 

 treatise on optics had been communicated at diflf'erent times to the Royal 

 Society; besides the experiments on refraction and the theory of the rain- 

 bow, they consist of an elegant analysis of the colours of thin transparent 



