Prof. Dove on the Dichrooscope. 353 



experiments, or a circular aperture for grating-experiments. Two 

 mirrors belong to the apparatus, 108 millims. long and 60 millims. 

 broad, one of which is silvered, and the other blackened. Either 

 of these mirrors can be placed at g in the polarizing angle, by- 

 means of a cleft in the direction g e ; in this case the polarizing 

 mirror c d is removed. For the future the mirror at g will be 

 designated by c d. 



The apparatus is intended either for ordinary daylight, or for 

 sunlight directly incident on c d at the polarizing angle. 



In order to simplify the phenomena of the different com- 

 binations, I assume that the linear-analysing arrangement is so 

 placed, that in a calc-spar cut at right angles to the axis the 

 rings with the black cross are visible. It is assumed that the 

 ocular is at right angles. If the plate of a body with feeble 

 double refraction, or if a plate of crystal with a large axial angle 

 is to be viewed, the polarizing microscope [Farbenlehre, p. 209) 

 is used as an analysing arrangement. It is so arranged that the 

 mica plate which serves for circular or elliptical analysis can be 

 used in the same manner as the ordinary ocular. If, on the 

 contrary, cooled or pressed glasses, or crystals, are viewed beyond 

 the distance of perceptible vision, the ocular is removed on one 

 side, and the ordinary analysing Nicol is used. * 



The following combinations are obtained : — 



l.cd silvered mirror, ef the glass disc. Natural light reaches 

 the analysing arrangement from c d } and linear-jpolarized from ef. 



(a) fg closed by the slide ; white light linear-polarized. 



(b) fg closed by the. slide, in hf coloured glass ; according to 

 the nature of the glass, monochromatic or polychromatic linear- 

 polarized light. 



(c) Without slide and without coloured glass ; white natural 

 light and white linear-polarized light, consequently partially 

 polarized, the rings scarcely visible. If, after looking for some 

 time at the rings with the black cross, the slide at fg is rapidly 

 removed, the complementary rings with the clear cross are seen 

 at first. 



(d) Without slide, the coloured glass at fg ; combination of 

 unpolarized coloured light with white linear-polarized light. 

 The cross brightly coloured in the colour of the glass ; the rings 

 in white light are somewhat changed by the uniform coloured 

 light. 



(e) The coloured glass at hf without slide ; coloured polarized 

 light with white unpolarized. The rings almost invisible, on 

 account of the preponderating white light. 



(/) Different-coloured glasses at hf and gf The system of 

 rings appears as if the analysing arrangement had been turned 

 90° 3 the cross is coloured, and the rings appear as changes from 



