POLARISED LIGHT. 



143 



any colour, there is only a slightly olive-green tinge ; but 

 if two crystals, crossing at right angles, be examined, the 

 spot where they intersect appears perfectly black, even if 

 the crystals are not one five-hundredth of an inch in thick- 

 ness. If the light be in the slightest degree polarised — as 

 by reflection from a cloud, or by the blue sky, or from the 

 glass surface of the mirror of the microscope placed at the 

 polarising angle 56° 45' — these little prisms immediately 

 assume complementary colours : one appears green, and 

 the other pink, and the part at which they cross is a 

 chocolate or deep chestnut-brown, instead of black. As 

 the result of a series of very elaborate experiments. Dr. 

 Herapath finds that this salt possesses the properties of 

 tourmaline in a very exalted degree, as well as of a plate 

 of selenite ; so that it combines the properties of polarising 

 a ray and of depolarising it. Dr. Herapath has succeeded 

 in making artificial tourmalines large enough to surmount 

 the eye-piece of the microscope; so that all experiments 

 with those crystals upon polarised light may be made 

 without the tourmaline or Nicol's prism. The brilliancy 

 of the colours is much more intense with the artificial 

 crystal than when employing the natural tourmaline. As 

 an analyser above the eye-piece, it offers some advantages 

 over the Nicol's prism in the same position, as it gives a 

 perfectly uniform tint of colour over a much more exten- 

 sive field than can be had with the prism. ^ As these 

 crystals are liable to be injured by damp, and thus lose 

 their polarising property, when out of use they should be 

 kept in a dark dry place. 



" The following experiments, if carefully performed, will 

 illustrate the most striking phenomena of double refraction, 

 and form a useful introduction to the practical application 

 of this principle. 



(1) Dr. Herapath has given a later and better process for the manufacture of 

 these artificial tourmahnes in the Quarterly Journal of Microscopical Science for 

 January, 1854. " These beautiful rosette crystals are best made as follows : — 

 Take a moderately strong solution of Cinchonidine in Herapath's test-fluid (as 

 already described). A little of this is dropped on the centre of a slide and laid 

 down for a time, until the first crystals are observed to be formed near the 

 margin. The slide should now be placed upon the stage of the microscope, and 

 the progress of formation of the crystals closely watched. When these are seen 

 to be large enough, and it is deemed necessary to stop their further development, 

 the slide must be quickly transferred to the palm of the hand, the warmth of • 

 which will be found sufficient to stop further crystallization." 



