326 M. Pouillet on the Recent Researches o/'Prof. Faraday. 



is a system of two juxtaposed plates of quartz cemented by 

 one margin, and worked together in order to fulfill a double 

 condition of giving them exactly the same thickness and ren- 

 dering each perfectly perpendicular to the axis. The surface 

 of junction of these plates being parallel to the pencil of light 

 and occupying the centre of its breadth, it is evident that the 

 first half of the pencil traverses one of the plates only, and 

 the second half the other plate ; and as they were selected of 

 opposite rotatory power, the first half of the polarized pencil 

 is found to have its planes of polarization deviated, for instance 

 towards the right, by a certain angle, and the second half, on 

 the contrary, has its planes of polarization deviated towards 

 the left by a perfectly equal angular magnitude. The magni- 

 tude of these deviations depends on the common thickness of 

 the two plates, which is usually from five to six millimetres. 



The ocular portion, or that directed towards the eye, con- 

 sists, in the first place, of a thick plate of rock-crystal, like- 

 wise perpendicular to the axis, having for instance a rotatory 

 power to the right, and a thickness of five millimetres very 

 accurately determined by the spherometer. Behind this plate 

 is the compensator, composed of two equal prismatic plates, 

 provided with a similar rotatory power towards the left, i. e. 

 in a contrary direction to the first. These two prisms, op- 

 posed like two wedges by their acute angle, are moved simul- 

 taneously by the same spring ; they slide one upon the other, 

 to be arranged sometimes by their less sometimes by their 

 greater thickness, and thus always form a system equivalent 

 to a parallel plate, but one which would vary from the thick- 

 ness to nearly double that of the base of each prism. To 

 avoid the deviations which the light might experience from 

 the variable distance of these prisms and the obliquity of the 

 surfaces, each one is compensated by a glass prism. 



Lastly, behind the compensator is a doubly refracting 

 achromatic prism and a small Gallilean telescope, to which 

 the eye is applied to observe the pencil of light which has 

 passed both the objective portion, the intermediate bodies 

 submitted to examination, and the ocular portion of the 

 instrument. 



The graduation of the compensator is easily made; and when 

 this has once been done with sufficient care, the instrument 

 indicates that the cause, whatever it be, which produces the 

 deviation in the plane of polarization has an intensity equiva- 

 lent to that of a plate of quartz of a known thickness ; always, 

 be it understood, on the condition that this cause exercises 

 on the various simple lights actions comparable to that which 

 the quartz exercises. 



