In the year he published a very interesting me moir on the duration of the impression of light upon the retina : He found, that, during a dark night, the sensation produced by a lighted coal continued for eight thirds of time to maintain its impression upon the re tina. But that if a circle, with an aperture in it, was moved about its centre, and a lighted torch placed be hind the circle, the torch would continue visible for nine thirds of time. The experiments of D'Arcy, ingenious though they be, were too limited to extend our views in this branch of physics. This subject is very imperfectly understood ; and accurate experiments are still wanting to enable us to determine the duration of the impression, when the light has different colours and different de grees of intensity, and when the time of its action is. varied. The subject presents several curious anomalies•: The impression of the solar image will often continue for 10 minutes, and longer upon the retina ; and the period of its continuance is so variable, that it will often. vanish and re-appear in the most capricious manner_ See ACCIDENTAL COLOURS and SPECTRUM, where many new experiments on this subject will be found.
In the year 1734, M. D'Arcy published an useful me moir on hydraulic machines. Deparcieux had found, from experiment, that overshot wheels produce an effect proportioned to the slowness of their motion. M. D'Arcy, on the contrary, maintained, that there was a determinate velocity when the effect of the wheel was a maximum ; and, by a comparison of Deparcieux's experiments with his own lOrmuloe, he has sliewn that the overshot wheel employed by Deparcieux never moved with such a small velocity as corresponded with the maximum effect, and that if he had increased the diameter ol his cylinders, or the size of his weights, he would have found, from his own experiments, the degree of velocity which gave a maximum effect.
D'Arcy went to Ireland in 1767, and one of his uncles offered him a considerable fortune, if he would continue in his native country. This alluring offer, how ever, he rejected ; and he returned to France, by the way of London, where he was received with that re spect which was justly clue to his talents and character.
In 1770 he was made field marshal, and also pension ary ol the academy ; and, some years afterwards, he inherited a large fortune, left him by the uncle under whose care he had received his education. In 1777 he married his niece, and took the title of the count D'Arcy ; but this union, which promised him much happiness, was soon dissolved. He was seized with a cholera mor bus, and died on the October, 1779.
The count D'Arcy was of the middle stature, and was distinguished by his fine commanding figure. His tem per was ardent, his mind penetrating and active, and his soul ambitiously devoted to the pursuit of science. The gaiety of his life however, the professional interruptions which he experienced, and his limited fortune, prevent ed him from executing the various experiments which he had conceived. His experiments are all marked by fertility of invention, and his writings are distinguished by profound and ingenious views. His name will be associated with these of Euler, Bernouilli, D'Alembert, and Clairaut, and will long adorn the history of science.
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