the Osteoscope, and the Telephone. 455 



It is evident that, under the conditions of u otheoscope, No. 1," 

 described in p. 177 of No. 180 of the ' Proceedings of the 

 Koyal Society,' the action of the light of a candle on the fixed 

 plate blacked on one side is just the same as that on a vane, 

 blacked on one side, of the revolving radiometer ; but as 

 motion of the plate is prevented, there is a reflux of the aethe- 

 real current — the effect of which, judging theoretically, I 

 should expect to be like that of wind emanating from the 

 black surface, and (in accordance with Mr. Crookes's descrip- 

 tion) " blowing away whatever easily movable body happens 

 to be in front of it, irrespective of colour, shape, or material," 

 causing thus the rotation of the contiguous four-armed fly. 

 In this respect the action resembles that of a body electrified 

 by friction on small particles of any kind in its neighbourhood, 

 which, in fact, under particular circumstances has been named 

 electric wind. 



Experiment has shown that a radiometer will rotate under 

 the influence of light when both sides of each vane are equally 

 bright, if only the vane be cup-shaped — that is, convex on one 

 side and concave on the other. This is a most remarkable 

 fact, and may be regarded as an eaperimentum cruris, by which 

 the truth of any proposed theory of the rotation of the radio- 

 meter may be tested. As far as I am aware, no other attempt 

 has been made to account for this fact than that which I have 

 produced in the Number of the Philosophical Magazine for 

 April 1877, supplemented by some remarks in a letter ad- 

 dressed to the Editors in the Number for May 1877. The 

 explanations there offered depend on the same principles of the 

 hydrodynamical theory of physical force as those applied in 

 the explanations of the motions of the radiometer and the 

 otheoscope, with the addition of considerations respecting the 

 variation of effect of the incident light at different points of the 

 surface of the radiometer as depending on the convexity or 

 concavity. The theoretical result is, that the radiometer ro- 

 tates as if pushed on the convex side, which agrees with expe- 

 riment. 



As respects the Telephone, an essential part of the explana- 

 tion of its action is furnished by the general Theorems before 

 enunciated. In the ordinary use of this instrument the voice 

 of the speaker, by setting an iron plate in vibration, is sup- 

 posed to generate electric or galvanic currents, which being 

 intensified as to their efficiency, and conducted, by known 

 experimental methods, are made to act on another plate at a 

 distant position, causing it to vibrate in such manner as to 

 produce by agitating the contiguous air, sounds that correspond 

 with more or less exactness to the pronounced words of the 



