POLARISED LIGHT 357 



position rotated 90 will equal their difference. If the foun- 

 dation be a star of differently coloured points, or with the 

 points arranged with polarising axes in different directions, 

 the result of rotating an even tint superposed will be very 

 different on each. Or such an even film may be rotated over 

 a film ground concave to show Newton's rings. If it equal 

 the thickness at about the middle of the radius, when crossed 

 the ring at that point will be black in the dark field ; and on 

 rotating the film, the rings will change colours in a beautiful 

 manner. The same will be the case with mica preparations 

 built up of concentric squares or circles successively smaller 

 in size (fig. 200). 



For crossing, superposing, and rotating with other pre- 

 parations, superposition films giving a 

 good red and green, and a good blue 

 and yellow, should be provided ; and 

 also a half- wave plate, which gives the 

 complementary in any case e.g. the dark 

 field becomes light and the bright field 

 dark, when a half- wave plate is employed. 

 This is ingeniously applied in two selenite 

 designs which are very amusing. In 

 one, the bust of a lady is composed of a nearly half- wave 

 film, with the result that when the analyser is rotated 90 a 

 fair woman is metamorphosed into a dark mulatto ; in the 

 other, the figure of a miller with a sack of flour, becomes 

 apparently a sweep carrying a bag of soot. 



Home-made preparations should be mounted in their 

 mahogany frames with rather stale putty, in which is mixed 

 a little red powder of some sort. 



206. Crystallisations. Substances which can be crystal- 

 lised in thin films on glass plates, if managed so that the 

 films are of suitable thickness, give magnificent projections. 

 Watery solutions are generally flowed over the discs, and in 

 many cases, e.g. potassic chlorate, a little gum arabic in the 



