854 Prof. Dove on the Dichrooscope. 



the colour of the two glasses ; for the parts which arc dark in 

 homogeneous light are illuminated by the coloured light from fg. 



(g) If a large rotating mica plate be placed at e h, the cor- 

 responding combinations of circular and elliptical light with 

 unpolarized light are obtained. 



2. The silvered mirror is replaced by the polarizing mirror. 

 There reach the analysing arrangement two masses of light 

 linearly polarized in the same plane, or if the mica plate is placed 

 at h e, circularly or elliptically polarized. They are, • 



(a) Both white or both coloured, if at hf and gf there are 

 either no glasses at all or both of the same colour (as in 1 a, b.) 



(b) White and coloured, if at hf or fg there is a coloured 

 glass, by which the white preponderates so greatly that the 

 action of the coloured almost disappears. 



(c) Different-coloured, if at hf and hg there are glasses of 

 different colours, in which case, strictly speaking, the plane 

 glass must not be parallel to the mirror, but each must be in- 

 clined at the maximum polarizing angle which corresponds to 

 that of the colour. 



If with a polarizing apparatus without a dichrooscope the light 

 of a white flame be concentrated on the polarizing Nicol, and if a 

 piece of cobalt glass 6 millims. thick be placed before the eye, 

 the blue and red rings in carbonate of lead are obtained quite 

 distinct*, but intersecting one another; in calc-spar splendid 

 alternations of deep-red, blue, and violet concentric circles are 

 obtained. By adding a green glass the blue rings may be 

 isolated, by adding a red glass the red rings. But cobalt glasses 

 which completely extinguish the middle of the spectrum are ex- 

 tremely rare ; and the darkening of the light is so great, that in 



* Both in this case, and also where, instead of a thick cobalt glass, blue 

 and red are combined in the dichrooscope, it may appear surprising that the 

 dark rings in blue light appear much more elongated in the direction of the 

 line joining the middle points of both systems than those in red light, 

 although the axial angle in red light is greater than in blue light. The 

 reason of this phenomenon is at once seen from the prism analysis ; the 

 spectrum of the flame seen through the cobalt glass appears to consist of 

 two masses of light separated by a dark space, — of which the red is homo- 

 geneous, for the form of the slit is very distinct ; while on the contrary the 

 blue mass of light extends over a larger space, and passes from light blue 

 to dark blue. The rings produced by this light are not simple, but appear 

 like the circular waves which are formed by drops falling in water after one 

 another in a straight line. Just as in this case two straight waves some- 

 what inclined to each other result from the simultaneously produced ele- 

 mentary waves, so in the case of the light, dark lines are formed which 

 accordingly appear rectilinear on the side, while at one end they are limited 

 by a flatter curve than at the other. This formation becomes distinct if a 

 green glass is combined with the cobalt glass. With prism analysis the 

 blue mass of light appears narrower, and in the polarizing arrangement the 

 rings were rounded. 



