108 ANNUAL OF SCIENTIFIC DISCOVERY. 



NEW METHOD OF PROPAGATING LIGHT. 



"A NEW MODE OF PROPAGATING LIGHT" is the title of a note 

 lately read by M. Babinet it a meeting of the Academy of Sciences at 

 Paris, in which he treats of the regular luminous waves which result 

 from a network or streaked surface placed in the path of a luminous 

 band. From this proceed many spectra of great brilliancy, anterior 

 and posterior, the origin of which cannot be derived either from prop- 

 agation in a straight line, or from reflection, refraction, or diffraction. 

 The very regular wave from the network borrows each of its elements 

 from the waves which successively arrive at the network, and thereby 

 obtains characters quite exclusive. The compensation in the celebrat- 

 ed experiment of Arago, which, according to Fresnel, prevents the in- 

 fluence of the motion of the earth from becoming sensible in the 

 phenomena of the prism, has no place in regard to the network ; and 

 M. Babinet concludes that, by substituting the network for the prism, 

 we shall be able to render sensible that influence, so long and so un- 

 successfully sought for by Fresnel and himself. 



COLOR OF WATER. 



The color of water has been frequently discussed by physicists. 

 Arago said, " The reflected color of water is blue, and the trans- 

 mitted color is green ; " and explained " the green color of the waves 

 by considering them as prisms of water, of which one of the faces re- 

 flects white light, which is refracted by the following wave, and thus 

 goes forth green." Bunsen asserts that water chemically pure is not 

 colorless, but is of a pure blue color. M. Wettstein, after minute 

 chemical researches, states that the green color is due to the presence 

 of organic matters. M. Beetz has recently investigated the subject, 

 and records his interesting observations in the Bibliotheque Universelle 

 of Geneva, the results of which are opposed to the conclusion of Arago, 

 and favor the opinion of Wettstein that the color is due to minute 

 particles of matter suspended in the water, modified by the color of 

 the sky and surrounding objects. 



THE GREAT FOUCAULT TELESCOPE. 



M. Leon Foucault has laid before the French Academy an account 

 of the great telescope constructed upon his principle for the Observa- 

 tory, at Paris. He observes that his efforts to obtain large instruments 

 with reflectors of silvered glass could not be deemed completely suc- 

 cessful until he had reached dimensions exceeding those of the largest 

 achromatic objectives, and that it was only by way of establishing a 

 claim to the recognition of his plans that he announced the formation 

 of mirrors of 10, 20, and 40 centimetres in diameter. Now, he is able 

 to speak of one nearly 80 centimetres in diameter, having a focal 

 length of 4| metres, 1 which has been completed in the establishment 

 of M. Secretan. This mirror, mounted in a Newtonian telescope, has 

 been at work for some months, at the Observatory, performing to the 

 entire satisfaction of the director, M. Chacornac. 



iThe metre is 39.3779 inches ; the centimetre 0.3937 of an inch. 



