272 POLARISED LIGHT AND ITS APPLICATIONS 



two 3 by I slips, or one such slip and a cover-glass, are pressed 

 together so as to touch in the centre. 



It is to be observed that the colours obtaine'd with any thick- 

 ness of selenite in the two positions of the analyser are always 

 strictly complementary colours, since those colours which are best 

 transmitted in one position are quenched in the other position, and 

 vice-versa. This every microscopist knows, but it is most readily 

 verified experimentally by placing a piece of Iceland spar over a 

 hole in a piece of paper, laying a piece of selenite over the spar, 

 and examining the two images with an analyser. The colours of 

 the images will be seen to be complementary, but the part where 

 they overlap will still appear white (Fig. 2). 



The effect of the thickness of an object on its colour is well 

 shown in a slide of plaited horsehair. Where two hairs cross we 

 get a colour quite different from that of either hair. 



When two selenites are placed one above the other, we get a 

 different colour from that of either selenite seen separately. A 

 second selenite, placed with its optic axes parallel to the corres- 

 ponding axes of the first, will evidently have the same effect as 

 increasing the thickness of the first selenite by that of the second. 

 On the contrary, if the second is turned through a right angle so 

 that its optic axes are now perpendicular to the corresponding axes 

 of the first — {i.e., so that the axis of the extraordi?tary ray in one 

 selenite is parallel to the axis of the ordinary ray in the other) — 

 it will, so to speak, hurry up those components of the waves that 

 had got most behind in the first selenite. The effect is the same 

 as if a slice of the same thickness as the second selenite had been 

 removed from the first. 



By rotating one selenite into different positions over the other, 

 we can get the colours corresponding to any thickness of selenite 

 not greater than the sum nor less than the difference of the thick- 

 nesses of the two selenites employed. This principle is used in 

 the selenite stages which accompany most of our more expensive 

 microscopes. 



The Black Cross seen on starch-grains is familiar. A similar 

 cross is seen on transverse sections of animal hairs, plant scales, 

 especially those from the leaves of certain species of the order 

 Eleagnacece — such as our Sea Buckthorn {Hippophae rhamfwides), 



