Mechanical Science. 395 



currents of ordinary electricity for -violent discharges of powerful 

 batteries, and for electro-chemical currents sufficiently powerful to 

 ignite and fuse them. Currents moving in the same and in opposite 

 directions have been used. The ray of light has been received on 

 the edges of diffringent plates connected with powerful magnets, 

 yet without any appreciable alteration. The rays of light, before 

 arriving at the diffringent body, have themselves been traversed by 

 powerful flames, by strong electrical curreuts and discharges, with- 

 out any change being perceived in the fringes and other appearances 

 of diffraction. The obscure bands in the shade of these wires have 

 remained invariable both in intensity and dimension. 



From these experiments, M. Haldat thinks, that the explanation 

 of diffraction, founded on an attractive force, or on certain atmos- 

 pheres supposed to exist around the body, can hardly obtain the 

 assent of philosophers, when this attractive force and these atmos- 

 pheres being submitted to agents so fitted to alter them, have pro- 

 duced no change in the phenomena. These facts certainly do not 

 establish the theory of undulations, but they help to destroy the only 

 explanation which can be opposed to it. The author, however, 

 does not hide the difficulties which these experiments also present 

 to the theory of undulations, and inquires how it is possible that 

 the motion of the luminous waves, which ought to be so regular 

 and constant, is not disturbed by the flowing out of subtile fluids 

 which strike against them in their course. He refers the solution 

 of this question to the period when science shall have penetrated 

 more intimately into the nature of those agents which at present are 

 known to us only by their effects. — Ann. de Chimie, xli. 424. 



6. On the Impressions produced by Light on the Eye. — The fol- 

 lowing are the conclusions to a memoire on this subject by M. 

 Plateau. First section — i. Any sensation of light whatever requires 

 an appreciable time for its complete formation, and also the same 

 time for its complete disappearance, ii. The sensations do not dis- 

 appear suddenly, but gradually diminish in intensity, iii. As a 

 sensation fades, the progress of its decrease is slower as the effect 

 is nearer to a close, iv. Different colours illuminated by daylight 

 produce sensations differing little from each other in their total 

 duration. Their order, in this respect, beginning with that which 

 produces the longest sensation, is white, yellow, red, blue. v. The 

 total duration from the time when the sensation has acquired its 

 fullest power, to that when it is hardly sensible, is very nearly 0."34. 

 vi. Finally, it results accidentally from the experiments, that the 

 principal colours arranged according to the intensity of the sensations 

 which they are competent to produce, stand in the following order, 

 white, yellow, red, blue. 



Second Section — i. New proofs confirm the order of colours con- 

 tained in the sixth result of the first section, ii. The visual angles 



OCT.— dec. 1829. 2 D 



