ON THE HISTORY OF OPTICS. 377 



by the shortest possible way ;* and Barrowf derived the same law, in a 

 more geometrical manner, from a similar hypothesis respecting the velocity 

 of light, by considering a pencil of light as a collection of collateral rays 

 influencing each other's motions. We are indebted to this learned mathe- 

 matician for the first accurate investigation of the properties of refracting 

 and reflecting surfaces, and for the most general determination of the situ- 

 ations of focal points. 



The industrious Mr. Boyle J had noticed with attention the phosphores- 

 cence of diamonds, the colours produced by the effect of scratches on the 

 surfaces of polished metals, and the diversified tints which a bubble or a 

 film of soapy water usually assumes. His assistant, Dr. Hooke, investi- 

 gated thse and other similar appearances with still greater accuracy, and 

 proposed, in his Micrographia, which was published in 1665, a theory of 

 light considerably resembling that of Descartes : he supposes that light is 

 an impulse propagated through a medium highly, but not infinitely, 

 elastic ; that refraction is produced by the readier transmission of light 

 through the denser medium, and that difference of colour consists in the 

 different law of the particular impulse constituting coloured light, so that 

 red and blue differ from each other in the same manner as the sound of a 

 violin and of a flute. He explained the colours of thin plates from the 

 interference of two such pulses partially reflected from the upper and under 

 surface; || but the hypothesis which he assumed respecting the nature of 

 colours, renders this explanation wholly inadequate, nor were the pheno- 

 mena 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 capable 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 

 interesting 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 

 accompany 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 



* See Maupertius, Hist, et Mem. de Paris^ 1744, p. 417, H. 53. 



f Lectiones Opticae, 4 to, Cantab. 1674. 



J Works, iii. 304. Microg. p. 56. 



|| Microg. p. 65. 



^ Physico-Mathesis de Lumine, Bonon. 1665. 



