356 Prof. Dove on the Dichrooscope, 



I have shown (Poggendorff's Annalen, vol. xxvi. p. 310) that, 

 on looking at a brightly illuminated aperture on a flame through 

 two crossed glass gratings held close before the eye, there is seen 

 the splendid phenomenon of the spectra distorted in the form of 

 quadrants, which Fraunhofer has depicted in the sixth plate of 

 his grating-experiments. 



These obliquely distorted spectra may be shown to be connected 

 with the apparent deflection of light depending on the length of 

 the wave, by interposing a monochromatic, dichromatic, or tri- 

 coloured glass, — in which cases the unchanged figures of the 

 aperture at regular distances are seen to form systems of qua- 

 drants with mutual edges ; in the first case there is one, in the 

 second two, and so on. In using the dichrooscope, the circular 

 aperture is in a slide at h e. It is viewed from the distance of 

 distinct vision, through gratings rotating on each other. The 

 alternate closing of hf and gf gives the components. 



In all these interference experiments, it is of course desirable 

 to know the homogeneous colours which prevail in the combined 

 action of the glasses. This is effected if the circular aperture is 

 changed for a narrow slit, and if a strongly refracting flint-glass 

 prism is used instead of the crossed or simple grating. In this 

 manner the component spectra and the result of their combina- 

 tion are obtained. 



3. Between the silvered mirror c d and the plane glass ef the 

 rotating mica plate is inserted. 



In this manner are obtained combinations of circularly or ellip- 

 tically polarized light with light linearly polarized by ef, which 

 reach the eye from gf. 



(a) If, without coloured glasses, the light incident through gf 

 is right-circularly polarized, and that through hf linearly polar- 

 ized, the distorted cross in the plate of calc-spar shows that the 

 light passing out is right-elliptically polarized light. 



(b) If, without coloured glasses, the light through gf is left- 

 circular, it gives with the linearly-polarized light incident through 

 hf left-elliptically polarized light. 



(c) If by turning the mica plate the light through gf is right- or 

 left- elliptical, it remains elliptical even if combined with the right- 

 or left-linearly polarized, but it approximates more towards linear, 

 and becomes so when the azimuth of the principal section is 0°. 



(d) If a coloured glass is inserted so as to colour either the 

 linear or the circular or elliptical light, the white light prepon- 

 derates so as to make it appear that the figure of the circular or 

 elliptical or that of the linear is seen in white light. 



(e) If, on the contrary, two coloured glasses are inserted, the 

 succession of colours on the rings of the straight quadrant 

 appears different and removed from those in the crooked one, 



