POLYCHROMATE 



589 



1635 



1634 



POINT N3ST is a style of lace formerly much in vogue, but now superseded by 

 the bobbin net manufacture. 



POLARISATION 1 OF LIGHT. It is not the purpose of the present -work to 

 deal with any of the peculiar phenomena of the physical powers, except so far as they 

 are involved in any of the processes of manufacture. Polarised light is employed in 

 the sugar refinery ; it therefore is necessary that some short account should be given 

 of the phenomena so called, and of the methods of rendering them available to useful 

 ends. ' 



Under the term Polarisation of Light is comprehended a variety of very singular 

 phenomena, which it is exceedingly difficult to explain within the space which can be 

 devoted to this article. For anything like an exact description of these peculiar and 

 striking phenomena, the reader is referred to works devoted specially to this branch 

 of science. For our purpose it will be sufficient to state that if a ray of light is 

 reflected from a plate of glass placed at an angle of about 56, it will be found to have 

 undergone a remarkable change. If the reflected ray of light is looked at through 

 a thin slice of Tourmaliiie, it will be found that while the ray is seen, when the 

 reflecting plate is in one position, it can no longer be seen through the transparent 

 crystal if the glass-plate is turned round 90, or if the crystal is turned to the same 

 extent ; although an ordinary ray of light is seen with equal intensity in whatever 

 position the crystal may be held. 



The ray of light by reflection, at or about the above-named angle, appears to have 

 assumed the position of a polar body, i.e., a 

 body having dissimilar sides, or it may be, 

 that the mode of motion has been altered 

 by the reflection at the polarising angle. 

 Light can be polarised by refraction equally 

 as well as by reflection. 



Under some circumstances, the condition 

 of Circular Polarisation is prodxiced. (See 

 Pereira's Lectures on Polarised Light.) We 

 do not attempt to explain this. The phe- 

 nomena alone are all we have now to deal 

 with. An instrument called a Polariscope is constructed upon 

 the principles shown in the accompanying figure (fig. 1634). 



If a ray of common light, a, be polarised by falling upon a 

 glass, b, at an angle of 56 45", the plane-polarised ray c, is 

 obtained. If this ray is transmitted through a pure solution of 

 crystallisable cane-sugar, and the ray as it emerges, e, be analysed 

 by a double-refracting rhomb of Iceland spar, /, two coloured 

 images are perceived, as shown in fig. 1635. One, o, is caused 

 by ordinary refraction, and the other, x, by extraordinary refraction. 

 g (fig. 1 634) is a lens to produce a well-defined image. The colours 

 of these images are complementary ; that is, when one is red the 

 other is green, when one is yellow the other is violet, when one 

 is blue the other is orange. By rotating the ' analyser,' the 

 rhomb of Iceland spar, the colours change. If the rotation be 

 right-handed, that is, as we turn a screw or cork-screw to make 

 it enter, the sequence of colours is red, orange, yellow, green, 

 blue, indigo, and violet, red. It will be understood that by 

 rotating the rhomb of Iceland spar, the extraordinary ray re- 

 volves around the ordinary ray, each undergoing a change of 

 colour. The sequence of the ordinary image being given above, 

 and the complementary colours named, it will be seen that the 

 sequence of colours on the extraordinary image will be green, \ 

 blue, indigo, and violet, red, orange, yellow, green. In one com- 

 plete revolution of the analyser each of the colours of the 

 spectrum occurs twice for each image. The polariscope is now used for both the 

 qualitative and quantitative analysis of sugar. Indeed, the minutest difference in 

 chemical character and physical constitution can be readily detected by this instru- 

 ment. See SUGAR. 



POXiXSHING-SXiATE. A grey or yellow slate composed of microscopic infusoria. 

 It is found abundantly in the coal-measures of Bohemia, and in the Auvergne. 



POLYCHROIVIATE. (JEsculine.) A compound from which a variety of colours 

 may be prepared. 



A great many vegetables give, when treated with hot water, a solution which 

 appears yellow by transmitted light, but blue by reflected light. The inner bark of 

 the horse-chestnut is a peculiar example of this. See FLUOHESCESCE. 



