DARCY. 53 This principle, which he again brought forward in 1750, by the name of “the Principle of the Preservation of Action,” in order to oppose it to Maupertuis’ principle of the least action, Darcy made use of in solving the problem of the precession of the equinoxes. In this attempt, however, he unfortunately failed; and in general, it is to be observed, that though a l l principles o f this kind, may b e used a s a mathematical formulae, two o f them a t least, must necessarily b e employed i n the inves tigation o f problems, and even these with great caution; s o that the luminous and simple principle given b y M. D'Alembert i n 1742, i s the only one, o n account o f i t s being direct, which can b e sufficient o f itself for the solu tion o f problems. Having given t o the world “An Essay o n Artillery,” i n 1760, containing many curious experiments o n the charges o f powder, &c. and several improvements o n Robins, who wrote on the same theories, and was not s o great a mathematician a s the subject o f the present memoir. I n 1765, h e published his “Memoir o n the Duration o f the Sensation o f Sight,” a work distinguished for ingenuity, and which shews him i n the best light, a s a n accurate and ingenious maker o f experiments. The result o f these re searches was, that a body may sometimes pass b y our eyes without being seen, o r marking i t s presence, otherwise than b y weakening the brightness o f the object i t covers; thus, i n turning pieces o f card painted blue and yellow, you only perceive a continued circle o f green; thus the seven prismatic colours rapidly turned produce a n obscure white, which i s the obscurer a s the motion i s more rapid. As this duration o f the sensation increases with the bright ness o f the object, i t would have been interesting t o know the laws according t o which the augmentation o f the duration follows the intensity o f the light o f a n object, which motion makes continually visible; but Darcy being now obliged t o rely upon the sight o f others, h e relin quished the pursuit. But being constantlyemployed i n
