248 Prof. H. F. Dawes on Image 



This gives, in order o£ approximation, 



F °4;> F o=4>i?' F.«£ + ^±ff(a* + M.W (22) 



In these p never occurs in the denominator, so that, as 

 might be expected, viscosity o£ displacement of charge would 

 increase the field required for breakdown as the frequency 

 is raised, and this is not the case. 



Dielectric hysteresis must therefore be considered as 

 neither a static nor a viscous retardation of displacement, 

 but as a quasi-elastic adjustment of equilibrium of the field 

 of interattraction between opposite charges in molecules 

 whose (fixed) axes are in perfect confusion of alignment ; 

 the molecules being partly polarized, and interattraction 

 started, by the application of the external field. 



XXX. Image Formation by Crystalline Media. By H. F. 

 Dawes, Professor of PJiysics, McMaster University, Toronto^ 

 Canada *. 



IX 1877 Stokes f discussed the question of the position of 

 the image (the " apparent depth " of an object-point) 

 due to the refraction of light from a crystal to air through a 

 cleavage-plane or other plane refracting surface. This paper 

 was written with the object of developing a method of testing 

 crystals by applying the then recently discovered microscope 

 method of determining the index of refraction, and was 

 tested out for a great many specimens by Sorby %. 



Stokes's investigation does not seem to have been very 

 generally followed up or applied from an optical point of 

 view, although the results are particularly interesting in the 

 study of the optical properties of crystals. In fact, the 

 only record of the application of the principles developed 

 which I have been able to find is in Clay's admirable 

 collection of Experiments in Light. Independent of these 

 sources, however, the writer developed in 1908 a simple 

 laboratory experiment of the type under consideration for 

 use in his laboratory classes at the Physical Laboratory of 

 the University of Toronto. In addition to the phenomena 

 considered by Stokes there are further cases of image 

 formation which are interesting and important, and the 

 writer proposes to consider certain of these. 



* Communicated by Professor J. C. McLennan, F.R.S. 

 + G. G. Stokes, Proc. Rov. Soc. xxvi. p. 385 (1877). 

 X Sorby, ibid. p. 384. 



