414 Prof. 0. Lodge on Absolute Velocity and 



that from which they were condensed, then it may be not 

 altogether out of place in the present lecture to speculate on 

 the relation between the actual light from the glowing clouds 

 and possible oscillations of the particles of the medium in 

 which they exist. There is no need to emphasize the idea 

 that the oscillations of very simple molecular systems give 

 rise to undulations which can only be perceived when by their 

 action upon something more complex than themselves they 

 cause either a distinct chemical change or set up undulations 

 within the range of the visible spectrum. 



May it be that there are similar oscillations in the sun, 

 that the simpler materials out of which the photospheric 

 clouds are condensed vibrate too quickly to give out visible 

 light, but are rendered visible when they are absorbed and 

 responded to by the more complex groupings of the condensed 

 masses? A sun-spot, looked upon as a partial absence of 

 clouds, would mean that the conditions which serve to screen 

 us to a great extent from the rapid undulations have been 

 somewhat modified. 



Is it too much to suppose, in view of the close resemblances 

 between many of the actions of light and electricity, and of 

 the well-known electrical effects of ultra-violet light and of 

 ^-rays, that the breaking down of a dielectric which they 

 can accomplish may, on a vastly larger scale, accompany an 

 unusual exposure of the earth to similarly rapid undulations ? 

 Should there be anything in this suggestion it may help to 

 remove a part of the difficulty in relating the presence of 

 sun-spots to those casual electrical disturbances with which 

 they undoubtedly coincide in point of time. 



XLII. On the Question of Absolute Velocity and on the 

 Mechanical Function of an ^Jther, with some Remarks on the 

 Pressure of Radiation. By Prof. Olivek Lodge, F.R.S.* 



UNDER the belief that fundamental doctrines of science 

 can be and should be treated in an exceedingly simple 

 manner as well as by abstruse processes, the following short 

 paper was mostly written a few years ago as a continuation of 

 an elementary treatment of dynamical foundations, and as a 

 prelude to a similar treatment of some more difficult problems 

 concerning which I thought at that time that I had some trust- 

 \\ orthy facts. It may be some time now before it is completed, 

 so I publish it as it stands at present. 



In the Phil. Mag. for February 1889, vol. xxvii. p. 115, 



Professor Newcomb calls attention to a fact that must often 



have struck people as a difficulty (Mr. Heaviside mentioned it 



twenty-five years ago in l The English Mechanic'), viz. that 



* Communicated by the Author, 



