POLARISED LIGHT. 153 



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. 1 These crystals 

 frequently lose their polarising property. AVhen out of 

 use they should be kept in a dark, dry place. Mr. Lobb 

 has had one in use three years, it is as good at the present 

 time as it was on the day he purchased it from Messrs. 

 Home and Thornthwaite. 



A variety of interesting phenomena have been described 

 by Mr. S. Legg, in the Transactions of the Microscopical 

 Society. He observes: 



"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. 



