534 REPORT— 1901. 



strip of silver. One of these tubes was put into a box in a dark room, and the 

 other was exposed to light in a conservatory. Here they were left for five years — 

 forgotten, I presume — and when examined at the end of that time it was found 

 that the silver strip in the tube exposed in the conservatory was very much 

 blackened, while that in the tube kept in the dark was scarcely discoloured at all. 



G. On the Necessity Jor Postulating an Ether. B. HoPKiNSOX. 



The difference between those who say that there must be a medium to transmit 

 gravitation and those who deny its necessity is a purely metaphysical one. All the 

 facts of gravitation can be described or expressed without any reference to a 

 medium. In like manner it would appear so far as terrestrial phenomena go that 

 the facts of light transmission can be so described, in which case the necessity of an 

 ether for conveying light is again purely metaphysical. We may say that a 

 luminous body A causes a disturbance at P in its neighbourhood ; the disturbance 

 is properly represented by a vector at right angles to the line joining A to P, and 



its amount is -~~^ — -, where t is the time, r the distance AP, and V a velo- 



city. Aberration is expressed by saying that if A be in motion relative to P, in a 

 direction at right angles to AP, the disturbance experienced at P is the same as 

 that which would be produced by a similar luminous body at A' at rest relative to 



P, where — -=- = J- - and AA' is in the direction of motion of A. There 



AP V 



is here no mention at all of a medium, but a complete account is given of the 



cardinal optical phenomena. 



This mode of expressing the facts, however, fails to cover the phenomena of 



spectroscopic double stars. The periodic doubling of lines in their spectra shows 



these stars (apparently single as seen in a telescope) to consist of two components 



moving one about the other with an orbital velocity which can be computed from 



the displacement of the lines. When the two components are in the line joining 



the star to the earth, there is no doubling of the lines, but one component is 



moving to the right and the other to the left with this orbital velocity. Now 



according to the above-stated expression for aberration, or any expression which 



only involves the motion of source and receiver relative to each other, the two 



components should, when in the line of sight, be apparently separated owing to 



the difference of their motions relative to the earth. The angular amount of the 



„ , , J , , twice orbital velocity 



separation oi the two components would be equal to , — : an 



. velocity of light 



amount sufficient in some cases to be visible to the naked eye. The star would in 



fact appear periodically to be double, the doubling occurring alternately with the 



displacement of the lines in the spectroscope. Since no such doubling takes place 



we infer either that aberration cannot be completely expressed in terms of 



relative motion of source and receiver, or that the accepted theory of these stars 



is wrong. The former alternative, which seems the more probable, forces one to 



recognise a something other than matter to which the motion of matter can be 



referred. In fact, it may almost be said that in this way the ether is made 



manifest to our senses as having position. This reason for postulating an ether 



differs in kind from the metaphysical reasons usually advanced; it may be 



described as furnishing a logical necessity for an ether. 



7. On the Change of Conductivity of Metallic Particles under Cyclic 

 Electro-motive Variation. By Professor Jagadis Chunder Bose, 

 M.A., D.Sc. 



(1) Under the action of electric radiation the conductivity of metallic par- 

 ticles exhibits variation. In the positive class, like iron, there is an increase, 



