temporarily produced in Isotropic Bodies, 347 



533*4 millionths of a millimetre, which thickness is very nearly 



equal to 2 7^. "" 



To construct the following table, I have chosen a piece of very 

 pure Clichy crown glass (borosilicate of zinc of the manufacture 

 of MM. Maes and Clemandot), which could be compressed in 



• the press A, until it gave a difference of path of 7 ^ ; beyond 



this limit the colours are too much mixed with white light, and 

 are consequently too pale to serve as measures. The charges 

 ! which it was found necessary to employ are as follows : — 



These figures are the means of a great number of concordant 

 measures ; they may be considered exact to about 1 kilogramme 

 for the experiments with the white light, and to 2 kilogrammes 

 for the experiments with the homogeneous orange light. 



The charge which produces the difference of path ^ is consi- 

 derably less than all which follow ; this same inequality is pre- 

 sented by all isotropic bodies, and we shall have to take it 

 into account ; but at present it is enough to prove that it ia 

 sufficiently small, and that the augmentation of the charge is 

 sufficiently regular, to permit of our finding by interpolation the 

 difference of path corresponding to any charge, and consequently 

 to any tint whatever. We have chosen in each ring a certain 

 number of tints well characterized and easy to reproduce, and 

 have determined the charges necessary to be applied to the same 

 piece of crown glass to produce them. Let "P^ be one of the 

 charges, and suppose that it stands between P^ which gives the 



double refraction measured by n -^, and Pj which corresponds 

 to («-f 1) ~; we have for this charge the difference of path 



This formula has served for the construction of the following 

 table, in which the second column d contains the difference be- 

 tween the paths traversed by the ordinary and extraordinary rays, 

 expressed in millionths of a millimetre. 



In the third column e, are inscribed the thicknesses of the 

 plates of air which transmit and reflect the colours inscribed in 



