144 THE MICROSCOPE. 



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 per- 

 fectly black, even if the crystals are not one five- 

 hundredth of an inch in thickness. 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 cho- 

 colate 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 extensive field than can be 

 had with the prism. 1 These crystals are liable to be 

 injured by damp. 



" The following experiments, if carefully performed, 

 will illustrate the most striking phenomena of double 

 refraction, and form a useful introduction to the prac- 

 tical application of this principle. 



(1) Dr. Herapath subsequently furnished a better process for the manu- 

 facture of these artificial tourmalines, see Quarterly Journal of Microscopical 

 Science for January, 1854. " These beautiful rosette crystals are 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 

 ft 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 

 trystallization," 



