112 THE MICROSCOPE. 



ifc. Dr. Herapath lias succeeded in making artificial tourmalines large 

 enough to surmount the eye-piece of the microscope ; so that all expe- 

 riments 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 advan- 

 tages over the Nicol's prism in the sante position, as it gives a perfectly 

 uniform tint of colour over a much more extensive field than can be 

 had with the prism."* 



A variety of interesting phenomena have been described by Mr. 

 S. Legg in the Transactions of the Microscopical Society. He says : 



" 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. 



A plate of brass, fig. 77, three inches by one, perforated with a 

 series of holes from about one-sixteenth to one-fourth of an inch in 



fig. 77. 



diameter the size of the smallest should be in accordance with the 

 power of the object-glass, and the separating power of the double re- 

 fraction. 



Experiment 1. Place the brass plate so that the smallest hole shall 

 be in the centre of the stage of the instrument ; employ a low power 

 (1^ or 2 inch) object-glass, and adjust the focus as for an ordinary 

 microscopic object ; place the doubly-refracting crystal over the eye- 

 piece, and there will appear two distinct images ; then, by revolving 

 the eye-piece, these will describe a circle, the circumference of which 

 cuts the centre of the field of view ; the one is called the ordinary, 

 the other the extraordinary ray. By passing the slide along, that the 

 larger orifices may appear in the field, the images will not be com- 

 pletely separated, but will overlap, as represented in the figure. 



* Dr. Herapath. has given a later and better process for the manufacture of these 

 artificial tourmalines in the Quarterly Journal of Microscopical Science for January 

 1854. Also, for further researches on its polarising properties, see Philosophical 

 Magazine, May 1855. 



