40 



Mr. J. A. Fleming on Magneto-electric [Mar. 8, 



ptying the method so as to obtain in a compact form and at a small 

 cost a dispersive power exceeding that of any spectroscope on the old 

 plan, and free from the defects inherent in a complicated instrument. 



In conclusion, I may allude to an application of the remarkable pro- 

 perty of the half -prism which may perhaps prove of practical use. From 

 what precedes it will be clear that the half -prism, as far as its magni- 

 fying-power is concerned, is equivalent to a combination of an object- 

 glass and cylindrical eyepiece, the peculiarity being that it magnifies the 

 angle between two pencils of parallel rays without affecting the paral- 

 lelism of the rays in each pencil. The half-prisms which have so far 

 been considered are constructed so as to give great dispersion, and the 

 sun is seen through them as if through a telescope with an object-glass 

 formed of a convex flint lens and a concave crown. But it is obvious 

 that an achromatic prism may be formed on the same principle as the 

 achromatic object-glass, giving cylindrical magnifying-power without dis- 

 persion. If two such prisms be crossed at right angles, one behind the 

 other, the magnifying-power will be the same in both directions, and the 

 combination will act as an achromatic telescope with the advantage of 

 great compactness, since the eye can be applied close to the second prism. 



"Whether, however, this advantage would compensate for the greater 

 quantity of glass required and for the loss of light is a matter for prac- 

 tical consideration ; and I therefore defer the discussion of the achromatic 

 half-prism till I have satisfied myself that it has some practical utility, 

 even though it may never replace the refracting telescope for general use. 

 Eoyal Observatory, Greenwich, 

 1877, Jan. 17. 



March 8, 1877. 



Dr. J. D ALTON HOOKER, C.B., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. " On Magneto-electric Induction in Liquids and Gases. — 

 Part I. Production of Induced Currents in Electrolytes." 

 By J. A. Fleming, B.Sc. (Lond.). Communicated by Prof. 

 Stokes, Sec.U.S. Received February 6, 1877. 



(Abstract.) 



This paper contains an account of an experimental inquiry into the 

 production of induced currents in liquids by magneto-electric induction. 

 Faraday examined one such case of induction, in which a conducting 



