PHYSICS. 3G3 



tions they conclude that blue travels faster than red by 1.8 per cent, of 

 the whole velocity. Since their result for the mean velocity is greater 

 than that of Cornu or Michelson^ Forbes draws the conclusion that it is 

 because the electric light is blue, and blue travels faster than red. 

 (Proc. Roy. Soc, May, 1881 ; Nature, June, July, 1881, xxiv, 135, 303.) 



Kayleigh has discussed the above results, raising the question whether 

 the velocity determined by the toothed wheel is really the group-velocity 

 or the wave- velocity as the above authors have supposed it to be, since 

 they give the difference between blue and red. He concludes that the 

 group-velocity is whatthe method determines. The accordance between 

 the i)hysical and astronomical methods seems to show that there can be 

 no such difference in. the velocities of the extreme rays as 1.8 per cent. 

 {Nature, xVugust, 1881, xxiv, 382.) 



Michelson has published a note in which he gives his opinion that if 

 the velocity of the red and blue rays differed by as much as one-tenth 

 of one per cent, the image of the slit in his experiments would not have 

 been white, but would have been spread out into a spectrum. He calcu- 

 lates that, as the total displacement in his experiments was 133 milli- 

 meters, a difference of velocity of 1.8 per cent, between the blue and 

 red rays would have given a spectrum 2.4 millimeters in length. Xo 

 such spectrum was observed. (iV^ffliitre, September, 1881, xxiv, 460.) 



Cornu has described several forms of i)hotometric and spectrometric 

 apparatus, which he has used in his researches. They all are founded 

 upon a property of lenses discovered and utilized by Bouguer ; i. e., that 

 the focal image, as to form, is independent of the size and shape of the 

 aperture of the lens, and as to intensity is proportional to the surface of 

 this aperture. One form of the apparatus he calls a micro-photometer 

 and another form a spectro-photometer. {J. Pliys., May, 1881, x, 189.) 



2. Reflection and refraction. 



Jacob has suggested a modified form of scale for use with reflecting 

 instruments generally. The graduated paper scale is trimmed off along 

 the lower edge of the divisions, and placed on a plate of glass, finely 

 ground, below the paper. The reflected image is received on the back of 

 the glass, and the coincidence of the center wire with the scale divisions 

 may be observed more accurately than by the common method. The 

 lamp and slit are placed on one side, and the beam reflected to the 

 galvanometer mirror by a right-angled prism. {Nature, April, 1881, 

 xxiii, 527.) 



Bertin has published an extended memoir upon magic mirrors. After 

 an introduction describing these mirrors, he gives a history of their im- 

 portation into Europe and of the experiments made with them; then 

 follows the theories proposed to account for their action, the experiments 

 of Govi confirming Person's theory, the artificial production of these 

 mirrors by himself and Duboscq, and some exceptional effects which 

 have been observed. {Ann. Ghim. PJnjs., April, 1881, V, xxii, 472.) 



