50 



GENERAL CHARACTERS OF PRECIOUS STONES 



double images of the needle. Contrary to the previous ease, this experiment must be 



performed in a lighted room. 



When a stone thus examined shows unmistakably double images the fact may be 



regarded as a decisive proof of the doubly refracting nature of the stone ; when, however, 



single images only are observed the stone cannot be stated to be singly refracting on these 



grounds alone, for stones which have only feeble 

 double refraction may give double images so 

 close together, or may be overlapping, that to 

 recognise the double character of such images 

 is a matter of considerable difficulty. 



The investigation by the direct method of 

 the kind of refraction possessed by a stone thus 

 requires a certain amount of skill, which is only 

 acquired by practice. On this account the re- 

 fraction of stones is often investigated by an 

 indirect method, which has the advantage of 

 being applicable to stones with rounded surfaces, 

 and also to small and irregular fragments of 

 material, neither of which could be used with 

 the method of direct observation. Further, 

 very small cut stones ai-e easily examined by 

 the indirect method, while their examination by 

 the direct method would present difficulties. 



The instrument used for the indirect observa- 

 tion of the singly or doubly refracting character 

 of a stone is known as the polariscope. A 

 simple form of this instrument, sufficient for the 

 present purpose, is shown, one-third' the actual 

 size, in Fig. 27. 



This consists of a wooden box, H, into the 



cover, pp, of which fits the circular object-carrier, 



I 00 ; the latter consists of a plate of glass in a 



-o , . i X. i- ■ n , brass setting, and may be easily rotated. From 



Polariscope for oDservation in parallel _ ° •' _ •' 



the box rises the vertical brass rod, mm, which 

 carries, on the horizontal arm, h, a Nicol's prism, 

 n, constructed of Iceland-spar. This is placed in the same vertical line with the centre of 

 00, and is capable of being rotated in the arm, h. In the box, H, is fixed, at an angle of 

 33" with the vertical, a sheet of unsilvered glass, ss, or better still, a large number of thin 

 glass plates arranged in a pile. The box also contains an ordinary mirror, tt, the inclination 

 of which can be varied by means of tfie wooden wedge, K. 



Rays of light from a clear sky enter the open side of the box, as indicated in the figure 

 by the dotted line, and are reflected from the mirror, tt, on to the glass plate, ss, at an angle 

 of 67° with the normal to the plate, whence they are again reflected in a vertical direction 

 through the object-carrier and the Nicol's prism to the eye of the observer. 



Ordinary daylight, after reflection from the glass plate, ss, at the particular angle 

 mentioned above, becomes endowed with special properties, and is said to be polarised. n 

 other words, the rays of ordinary light which strike the plate, ss, are reflected from it as rays 

 of polarised light, and as such reach the Nicol's prism, n. On rotating the Nicol's prism, it 



Fig. 27. 

 light. (One third actual size.) 



