ox THE POLARIZATION OF LIGHT! 249 



substances, whether animal, vegetable, or mineral, which, by 

 the unequal arrangement of their particles, possess the property 

 of double refraction, will, when placed between the prisms, 

 exhibit colours varying according to the otherwise unap- 

 preciable difference of density of their various parts, and 

 these differences may thus be distinguished and traced out 

 much more satisfactorily than by common light. " Should, 

 however," says Mr. Woodward,* « the doubly refracting 

 properties of the tissue be too feeble to produce a sufficient 

 difference of colour, the effect may be considerably increased 

 by placing the object on a plate of selenite of uniform thick- 

 ness, for which purpose a thickness capable of producing a 

 bright purple or hght blue colour will be found to afford the 

 most agreeable contrast, and, as a single plate, to be the most 

 generally useful." 



In the preceding description of the colours produced by 

 polarized light, those of which mention has been chiefly made 

 are the red and the green ; it must not, however, be imagined 

 that these are the only colours, for, in practice, it will be 

 found that not only every colour of the spectrum, but every 

 variety of tint of each of these primary colours, wiU be pro- 

 duced by variations in the thickness of the doubly refracting 

 substance, through which the polarized light passes; these 

 tints may be classified into seven orders, as was done by 

 Newton, when he ascertained the thickness of coloured plates, 

 and particles of air, water, and glass. Selenite, from the cir- 

 cumstance of its splitting easily into laminae, may be obtained 

 of all thicknesses, and films of intermediate thicknesses between 

 •00124 and '01818 will give all tints of colour between the white 

 of Newton's first order and the white arising from the mixture 

 of aU the colours. The same variety of tints may be produced 

 by placing the films one over the other; for this purpose, 

 Mr. Darker, who has paid considerable attention to the sub- 

 ject of polarized light, and to whose ingenuity we are indebted 

 for some of the most beautiful of our apparatus for exhibiting 

 certain phenomena in connection with this subject, has con- 

 structed for Dr. Leeson the instrument represented by fig. 158. 

 Op. at, p. 31. 



