246 Transactions of the 



II. — On a Silvered Prism for the Successive Polarization of Light. 



By J. W. Stephenson, F.E.A.S., Treasurer K.M.S., and 



Actuary to the Equitable Life Assurance Society, 



{Taken us read brfore the Eoyal Mickoscopical Society, May 1, 1872.) 



A PAPER on the " Successive Polarization of Light," by Sir Charles 

 Wheatstone, was read before the Eoyal Society last year, wherein 

 the author made known another means of producing successive 

 polarization by the reflexion of plane polarized light from a plate 

 of pohshed silver, and Mr. Spottiswoode delivered a lecture at the 

 Eoyal Institution on these experiments, in which he stated that 

 " if a ray of plane polarized light fall upon a metallic reflector, it is 

 divided into two, whose vibrations are respectively parallel and 

 perpendicular to the reflector, and the latter is retarded behind the 

 former by a difierence of phase depending upon the angle of 

 incidence. If the plane of vibration of the incident ray be inclined 

 at an angle of 45° to the plane of incidence, the two rays into 

 which it is divided have nearly the same intensity. At an angle 

 nearly 45^, which varies with the metal employed, but which is 

 perfectly definite, the intensities become accurately equal. And 

 further, if the angle of incidence have a particular value dependent 

 upon the nature of the metal (for silver 72^), the retardation will 

 amount to a quarter of a wave length. These two rays, on leaving 

 the reflector, will re-combine, and in the last-mentioned circumstances 

 become a circularly-polarized ray. Lastly, the direction of motion 

 in this circular ray will depend upon the side on which the original 

 plane of vibration is inclined to the plane of incidence ; if, when it 

 is inclined on one side, the circular ray becomes right-handed, then 

 when it is inclined on the other, it becomes left-handed." 



The purport of the present communication is to show how, by 

 instrumental means, this method of producing successive polariza- 

 tion can be made applicable to the microscope. 



The first essential condition of such an arrangement is that the 

 ordinary polarizer should remain in its usual position beneath the 

 stage of the microscope, and another is that the silver plate, from 

 which the plane polarized ray is to be reflected, should have a flat 

 and polished surface, and be, if possible, protected from oxidation. 



These conditions are fulfilled, by placing between the polarizer 

 and the stage, a truncated glass prism, having two similar acute 

 angles of such a magnitude that the plane polarized ray shall, on 

 entering, be refracted from its course, and thus become incident 

 on the hypothenusal side of the prism, previously silvered by the 

 sugar of milk process, at an angle of 72^, whence after reflexion it 

 will on emergence again be refracted, and resume the original 

 straight line on which it was previously moving in the axis of the 

 instrument. 



