

Prof. Dove on the Dichrooscope. 357 



while the cross becomes distorted and appears of the colour of 

 the circular (elliptical) light. 



4. The parallel glass is replaced by the polarizing glass, and 

 the silvered mirror c d inserted at g. 



(a) The glass is so placed that the light entering at fg and 

 then polarized by refraction is equal in intensity to that which 

 enters at hf, and is then polarized at right angles to it by re- 

 fraction. No figure is formed on the plate of calc-spar ; the 

 light is unpolarized. 



(b) The slide is gradually pushed forward at fg ; and by the 

 gradually increasing difference between the masses of light 

 polarized at right angles to each other, the unpolarized light 

 passes through the middle stage of partially polarized into com- 

 pletely polarized. The phenomenon is the same as if two flat 

 prisms of tourmaline, whose edges are parallel to the axis of the 

 crystal, were gradually pushed one over the other so as to form 

 a plate becoming continually thicker. 



(c) The slide at hf is gradually pushed forward after the one 

 at gf has been removed. The phenomenon obtained appears as 

 if two prisms of topaz (smoky quartz) parallel to the axis were 

 superposed on one another. 



(d) Coloured glasses are placed in hf and fg ; the phenomena 

 of dichroitic crystals are obtained as follows : — 



(a) The plate of calc-spar is removed, and the Nicol exchanged 

 for a doubly refracting achromatic prism. When a round aper- 

 ture is inserted at h e, two figures of it are obtained in different 

 colours, which by rotating the analysing prism, pass into one 

 another. This is the dichroitic lens. By using a Nicol, a 

 picture is seen to change its colour. If glasses of the same 

 colour are placed at hf and gf, and if the glass piece is arranged 

 so that the refraction and reflexion of the polarized masses of 

 light are of unequal intensity, two figures of the same colour, 

 but of unequal intensity, are obtained, and the emergent light is 

 partially polarized. This represents the crystals which are 

 improperly termed dichroitic, but which are so far connected 

 with them that, when used as an analysing arrangement, they 

 produce the phenomena of tourmaline to a feebler extent. If 

 the intensity and the colour are equal, the arrangement repre- 

 sents the plate of a doubly refracting crystal which has no 

 dichroitic properties. 



(/3) The calc-spar plate is inserted; and the phenomena are 

 obtained which dichroitic crystals exhibit when they are used in 

 the polarizing apparatus as an analysing arrangement. Strictly 

 speaking, that is the polarizing arrangement in this case which 

 is the analysing in the other, and vice versa ; but since, from 

 the law of reciprocity, one arrangement results directly from the 



