temporarily produced in Isotropic Bodies. 349 



of path has amounted to -, for then the ordinary image is com- 



pletely obscure ; and if the least trace of rotation existed, it would 

 manifest itself immediately in this image. The insensibility 

 remains the same, whether we establish the current in one or 

 the other direction, or interrupt it^ or even when we reverse the 

 poles of the magnet. 



It is to be observed, that the glass endowed with the most 

 energetic rotatory power is at the same time that in which the 

 birefracting power is most feeble ; the same observation applies, 

 among isotropic bodies, to alum and rock-salt_, and a similar 

 analogy appears to exist in bodies naturally doubly refractive : 

 the rotation is zero in Iceland spar, the birefractive power of 

 which is 0*175, while it is very sensible in quartz, which pos- 

 sesses the very feeble refractive power of 0-009. This is a point 

 which it will be necessary to take into account in subse- 

 quent researches ; for the present it is enough that we have de- 

 monstrated that this apparently direct action of the magnetism 

 upon the sether depends essentially on the constitution of the 

 latter, and that it can be annulled by forces purely mechanical. 



Applications of the temporary double refraction. 



The formula (1) contains all the quantities which exert an 

 influence on the phsenomenon of double refraction : thus far we 

 have made use of it to determine the value of the indices of ex- 

 traordinary refraction; but when those indices are once known, 

 the same formula can serve for the determination of any one of 

 the quantities contained in it. 



A. Determination of the force P; — Chromatic Dynamometer. 



The dynamometers designed to measure the effects of traction, 

 have, during the last few years, been carried to a high degree of 

 perfection by the labours of MM. Poncelet and Morin; but this 

 is not the case with regard to the measurements of pressures 

 exerted between two solid bodies ; no exact instrument has as 

 yet been constructed for the purpose ; we know nothing of the 

 useful effect of the commonest machines, such as presses, vices, 

 systems of levers, &c. 



The chromatic dynamometer (Plate II. fig. 3) seems to me to 

 supply this want ; after what has been said, it will be easy to 

 understand its construction and application. The essential part 

 is a plate of glass, perfectly transparent in the direction of its 

 length, of such dimensions that it can support very considerable 

 pressures, and blackened all round with the exception of the two 

 faces through which the operator is to look. This plate of 



