378 LECTURE XL. 



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 Dr. Hooke, and disliked the dispute so much, that 

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

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

 1672, he had sent them a description of his reflecting telescope, t 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 1615. The principal 

 parts of the treatise on optics had been communicated at different times to 

 the Royal Society ; besides the experiments on refraction and the theory of 

 the rainbow, they consist of an elegant analysis of the colours of thin trans- 

 parent substances, in which the phenomena are reduced to their simplest 

 forms, and of a collection of miscellaneous experiments on the colours pro- 

 duced in cases of inflection or diffraction. 



With respect to the nature of light, the theory which Newton adopted 

 was materially different from the opinions of most of his predecessors. He 

 considered indeed the operation of an ethereal medium as absolutely neces- 

 sary to the production of the most remarkable effects of light, but he denied 

 that the motions of such a medium actually constituted light ; he asserted, 

 on the contrary, that the essence of light consisted in the projection of mi- 

 nute particles of matter from the luminous body, and maintained that this 

 projection was only accompanied by the vibration of a medium as an acci- 

 dental circumstance, which was also renewed at the surface of every re- 

 fractive or reflective substance. 



In the mean time Bartholin had called the attention of naturalists and 

 opticians to the singular properties of the Iceland crystal, and had hastily 

 examined the laws of its unusual refraction. On this subject Huygens had 

 been much more successful : his analysis of the phenomena of the double 

 refraction is a happy combination of accurate experiment with elegant 

 theory ; it was published in 1690, making a part of his treatise on light, 

 the fundamental doctrines of which he had communicated to the Academy 

 of Paris in 1678. They scarcely differ in their essential parts from those 

 of our countryman Dr. Hooke, but the subject of colours Huygens has left 

 wholly untouched. Roemer had then lately made the discovery of the 

 immense velocity with which light passes through the celestial regions, by 

 observing the apparent irregularities of the eclipses of Jupiter's satellites ; 

 and Huygens readily admitted this property into his system ; although 

 Hooke, 1 1 by a singular caprice, professed himself more ready to believe 

 that the propagation of light might be absolutely instantaneous, than that 

 its motion could be successive, and yet so inconceivably rapid. The merits 



* Birch, iii. 10, 52. Ph. Tr. viii. 5084, 6086. 



t Ph. Tr. 1672, pp. 4004, 4032; 1673, p. 6087. 



i Optica Promote, 1663. 



Eskinard's Century of Optical Problems. 



|| Lectures of Light, in Waller's Life and Works of Hooke, p. 77. From a 

 passage in the Micrographia, p. 56, it is evident not only that Hoake was ready to 

 admit the fact of the finite velocity of light, when proved, but that he anticipated 

 both the manner of proof and the result. 



