488 Sir Oliver Lodge on the 



suitable density. This density is matched by rotation of the 

 second nicol before and after the interposition of the substance 

 under test. The difference of readings gives, of course, the 

 optical rotation for the part of the spectrum under test. 



In the instrument exhibited room is provided for the inter- 

 position of a 100 mm. tube for liquids, but there is of course 

 no difficulty in extending this to any desired amount. It 

 will be seen that one measures by reducing one half of the 

 field to the intensity of the other half which is already fairly 

 dark. Thus one would expect the sensitiveness to be somewhat 

 less than that obtained by the use of polarizers, in which one 

 half of the field decreases while the other half increases in 

 intensity. But it is an exceedingly useful addition to the 

 spectrophotometer and produces a polarimeter free from 

 some serious defects of most instruments excepting those with 

 the Lippisch and similar polarizers. Moreover it is, of course, 

 suitable either for use by monochromatic light or for the 

 measurement of optical rotations for different wave-lengths. 



I may add that a photographic negative is the best neutral- 

 tint medium with which I am acquainted, and is what is 

 employed when using the instrument as a spectro-polarimeter. 



XXXIX. The Density of the JEther. 

 By Sir Oliver Lodge *. 



Part I. 



1. rFYEE setherial constants, /ul and k, — the magnetic per- 

 JL meability and the electric inductivity of free space, 

 have hitherto been of completely unknown value, both as to 

 kind of quality and as to numerical magnitude ; though the 

 reciprocal of their geometric mean is well known to be a 

 velocity, the velocity with which the aether transmits trans- 

 verse waves of all lengths and intensities. 



2. For many reasons I have been accustomed to think of 

 Att/h (the ungeometrical part of a coefficient of self-inductance) 

 as of something which by legitimate analogy could be spoken 

 of as aetherial density, and of AlTt/k as of something which 

 by similar material analogy must be expressible as setherial 

 elasticity or rigidity : this kind of rigidity being probably 

 due to the aether's own intrinsic energy of constitution, and 

 being explicable on a recondite hydrodynamical basis, as 

 treated in various forms by Lord Kelvin. 



3. I have also adduced reasons (in ' Modern Views of 

 Electricity ; ) for regarding the aether as exceedingly, perhaps 

 infinitely, incompressible ; this view being based in the first 



* Communicated by the Author. 



