354 OPTICAL PROJECTION 



parabolic fringes, and across a plain flat film exhibits straight 

 interference bands crossing the spectrum. And just as the 

 prism showed bands in the spectrum of a film of mica too 

 thick to show any colour, so a slice of selenite or mica 1 mm. 

 to 2 mm. thick will show numerous bands crossing its 

 spectrum, though the image of the slice appears colourless. 



204. Coloured Designs. Since colour depends on thickness 

 of the film, studied variations will of course give corresponding 

 variations of colour. The simplest case is a film of selenite 

 split from common samples ; this is nearly always irregular, 

 and therefore shows variations. The Newton's rings caused 

 by concave grinding have been already referred to. A slice 

 ground into a wedge, of course exhibits straight bands. Many 

 slides can be procured prepared in selenite, exhibiting 

 coloured stars, butterflies, birds, flowers, 

 chameleons, church windows, &c., show- 

 ing complementary colours in different 

 positions of the analyser. 



I consider mica preparations, however, 

 more really instructive, and many of them 

 can be prepared by the demonstrator 

 himself, as this crystal splits more readily 



FIG. i99.-Mica Design into even films - They can be Prepared 

 in two ways. Such simple patterns as 

 fig. 199 may have their outlines partially cut through a 

 rather thick sheet of mica, and layers of variable thickness 

 split off and lifted by a fine needle-point. The design will 

 then be displayed in various colours, care being taken to 

 find the polarising axes, and mount the mica so that these 

 stand in the proper position. But a better plan is to split 

 the mica into thin even sheets, and cut out geometrical 

 patterns from these, to be superposed one on another (taking 

 care that the polarising axes of all the superposed micas lie 

 in the same direction, anless purposely differentiated). The 

 films are then cemented on one another between two glass 



