862 



M. G. Wertheim on the double Refraction 



dually ascend through the whole series of tints, and with a press- 

 ure of 26,000 we shall come to zero. Consequently, and with- 

 out adding anything to the optical apparatus, we can proceed 



7\ 

 anew to a negative double refraction of-^-, which would be ob- 



tained with a total pressure of 39,000 kilogrammes, and so on. 



This apparatus appears to me capable of being employed in a 

 great number of scientific experiments, and it would enable 

 engineers to graduate in a rigorous manner manometers, safety- 

 valves, and other instruments of measurement which are ha- 

 bitually applied. 



I will confine myself to citing a few examples. 



With two small screws turned by the fingers merely, we can 

 produce a pressure of 220 kilogrammes. 



To copy a letter with an ordinary copying-press, a pressure of 

 800 to 900 kilogrammes is usually exerted. 



A large press, in the workshop of M. Breguet, has given 

 the results contained in the following table : — 



We have denoted by P' the force which is applied to one arm 

 of the lever, and by P the effect produced ; the constant quan- 

 tity 6G*5, which is independent of the force P', represents the 

 inertia of the instrument and the friction in the screw which 

 corresponds to this inertia : if, after having applied any forde 

 whatever P', we remove the force, the pressure will not entirely 

 disappear ; a coloration still exists corresponding to a pressure of 

 66*5 kilogrammes, which evidently arises from the inertia of 

 the apparatus. We see, then, that this force, which is generally 

 neglected, may in certain cases become very considerable. 



Through the kindness of M. Morin I have been also able to 

 make a series of experiments with the great hydraulic press with 

 four cylinders, which is placed in the Conservatoire des Arts el 

 Metiers; it is furnislicd with a manometer which communicates 

 directly with the water of the press ; so that the theoretic and 

 effective pressures may be compared with each other. The 



