860 M. G. Wertheim on the double Refraction 



glass a, furnished on its two parallel surfaces with parallel cards 

 of vulcanized caoutchouc, is placed between two surfaces of cast 

 iron well planed and sufficiently thick; the inferior of these 

 plates b carries two tubes of brass, the interior surfaces of which 

 are blackened. The object-tube contains a Nichol d at the ex- 

 tremity nearest to the observer, and carries at the other end a 

 plate of white porcelain e, which is moveable in two perpendicular 

 directions, and which, consequently, can always be placed in a 

 position to be well illuminated. This arrangement has the ad- 

 vantage of permitting those rays only to arrive at the Nichol which 

 are sensibly parallel to the axis, and to cut away the internal re- 

 flexions, which are always prejudicial ; the other tube /carries a 

 birefracting prism g. These two tubes are mounted stiffly in the 

 grooves h, adapted to the inferior plate, so that by employing 

 plates of glass of different thicknesses, we can always place their 

 axes in the prolongation of each other, and at half the height of 

 the glass. The upper plate of metal i is altogether free ; it is 

 placed upon the last card, and serves simply to transmit to the 

 glass the pressure it receives, without being able by any friction 

 whatever to cause a loss of force. It will be further understood, 

 that the exterior diameter of the tubes ought always to be smaller 

 than the sum of the thicknesses of the horizontal plates. 



The principal sections of the Nichol and the birefracting prism 

 are parallel to each other, and form an angle of 45 degrees with 

 the vertical. 



We have now only to place this small apparatus between the 

 two surfaces on which the pressure is to be exerted. Whatever 

 be the nature of the force applied, whatever be the losses which 

 it may have sustained before producing the useful effect, the 

 measure of this effect will be always given by the simple inspec- 

 tion of the colours which exhibit themselves in the two images, 

 that is to say, by the value of d. We have, in fact, the equation 



p_ </.E.La 



1 -lo • 



The observer can free himself from all calculation : for this 

 purpose I have caused to be painted the forty-live colours con- 

 tained in our table, on a black ground, so that they may be seen 

 in the instrument ; by the side of each couple are inscribed the 

 corresponding values of P for two plates of glass, one of which 

 is destined to measure feeble pressures, while the other can sup- 

 port more considerable ones. The experiments present but one 

 difficulty, which is to be avoided by preventing the transverse 

 flexure of the glass ; for the latter would cause the coloured 

 bands of Fresnel to exhibit themselves, and then all exact deter- 

 minations would become impossible. This cause of disturbance 



