VOL. XXIX.] PHILOSOPHICAL TBANSACTIONS. 22Q 



An Account of some Experiments on Light and Colours, formerly made hy Sir 

 Isaac Newton, and mentioned in his Optics, lately repeated before the Royal 

 Society by J. T. Desaguliers,"^ F.R.S. N° 348, p. 433. 



The manner of separating the primitive colours of light to such a degree, 

 that if any one of the separated lights be taken apart, its colour shall be found 

 unchangeable, was not published before Sir Isaac Newton's Optics came abroad. 

 For want of knowing how this was to be done, some gentlemen of the English 

 College at Leige, and M. Mariotte in France, and some others, took those 

 for primitive colours, which are made by admitting a beam of the sun's light 

 into a dark room through a small round hole, and refracting the beam by a tri- 

 angular prism of glass placed at the hole. And by trying the experiment in 

 this manner, they found that the colours thus made were capable of change, 



* John Theophilus Desaguliers, a very ingenious and respectable philosopher, was bom in l683, 

 at Rochelle, from whence he was brought by his father, who was a minister, while an infant, on the 

 revocation of the edict of Nantes. He studied at Christ's College, Oxford, where he proceeded to 

 the degree of LIv.D. and where, in 1702, he succeeded Dr. John Keill in reading lectures on experi- 

 mental philosophy at Hart Hall. In 1712 he married and settled in London, where he was the first 

 who introduced the reading of public lectures on such subjects in the metropolis, which he continued 

 during the rest of his life, with the greatest applause, having the honour several times of reading his 

 lectures before the king and royal family. So that it is said he went through no less than 150 courses 

 of those lectures in London, besides some in Holland. In 1714 he was chosen F. R. S. of which he 

 proved a very useful and valuable member. He entered into priest's orders in 1717, and afterwards 

 enjoyed two livings; he was first chaplain to the Duke of Chandos, and after\vards to Frederick, 

 Prince of Wales. But natural and experimental philosophy was his chief occupation and pursuit; so 

 that in the latter part of his life, he removed to lodgings over the Great Piazza in Covent Garden, 

 where he carried on his lectures with great success till the time of his death in 1749^ at 66 years 

 of age. 



Dr. Desaguliers was a member of several foreign academies, and corresponding member of that of 

 France; from whence he obtained the prize proposed there, for the best account of electricity. And 

 he communicated a multitude of curious and valuable papers to the Royal Society, between the years 

 1714 and 1743, or from vol. 29 to vol. 42. Besides those numerous communications, he published 

 several excellent works of his own, as well as some translations of other useful books ; particularly, 

 his large course of Experimental Philosophy, 1734, in 2 large vols. 4to. being the substance of his 

 public lectures, and abounding with descriptions of the most usefiil machines and philosophical instru- 

 ments. Also his System of Experimental Philosophy, proved by mechanics, in 4to. 1719; his 

 Physico- mechanical Experiments^ in 8vo. 1717; his translation of Gravesande's Mathematical Ele- 

 ments of Natural Philosophy, in 8vo. 1720, and in 2 vol, 4to. 1747; also an edition of David Gre- 

 gory's Elements of Catoptrics and Dioptrics, with an appendix on Reflecting Telescopes, in 8vo. 1735; 

 containing some original letters that passed between Sir Isaac Newton and Mr. James Gregory, relat- 

 ing to those telescopes. 



Dr. Desaguliers left a son, who was in the Royal Artillery, where he rose to be a colonel of one 

 of the battalions, and had the rank of general in the army. He died in the year 1780. 



