Prof. Dove on the Dichrooscope. 359 



The conditions for circular polarization, that is, equal inten- 

 sity of two masses of light polarized at right angles to each 

 other, whose difference of phase is an odd multiple of a quarter 

 undulation, can, as is well known, be satisfied in two ways, — 

 either by two total internal reflexions in a single-refracting body, 

 or by refraction in a double refractor. The condition of equal 

 intensity is satisfied in the first case if the azimuth of thejplane 

 ©f reflexion with the primitive plane of polarization is plus or 

 minus 45°, and in the second case if that of the principal 

 sections is + 45°. "FresnePs rhombohedron serves for total 

 reflexion, or if the light is to remain in the axis of the instru- 

 ment, my reversion prism {Farbenlehre, p. 240) . In total reflexion 



the difference of phase after n reflexions is * ; using double- 



o 



refracting bodies, it is proportional to the thickness of the plate, 

 and to its double-refracting power. Airy accordingly splits a 

 plate of mica* until it has the required difference of phase, 

 while in Babinet's compensator the change is gradually pro- 

 duced by wedges of rock-crystal moved gradually over each 

 other. In both cases with an equal double-refracting power the 

 thickness changes. The thickness, on the contrary, remains 

 the same with a change in the double-refracting power if, as I 

 have shown, circular polarization is produced by pressing or 

 warming a glass plate between the polarizing and analysing 

 arrangement ; but since the length of waves is different for dif- 

 ferent colours, the condition of a determinate difference of phase 

 can only be simultaneously satisfied for a determinate colour ; 

 and it follows directly from the formulse for both the rays sepa- 

 rated by double refraction, that with increasing thickness of the 

 plate the difference in this respect between the various colours 

 increases in a corresponding degree; so that the same arrange- 

 ment in one part of the linear-polarized spectrum changes the 

 light into circularly polarized light, in another into linearly 

 polarized light, in others into light polarized at right angles, with 

 all transitions through right and left elliptical. I have shown this 

 in the experiments on circular polarization by allowing, by means 

 of a rotating prism, the individual parts of the spectrum to pass 

 over the aperture of a polarizing Nicol. In this case the system 

 of rings in calc-spar, and then all changes of form correspond- 

 ing to the various stages of polarization, are seen to pass in single 

 colours, from which the very complicated phenomenon in white 

 light finds a direct explanation. The dichrooscope gives another 

 derivation of the same phsenomenon. For if a plate of mica is 

 inserted at he, a blue glass at hf } and a red glass at #/, on covering 



* Darker uses a plate of gypsum, aud a combination of several in the 

 beautiful apparatus which he calls the retarding slide. 



