POLAKISCOPES.J 



UNDULATOBY FORCES. LIGHT. 



81 



There are no natural analogies which we can offer as 

 illustrative of such phenomena. It is true that we have 

 magnetic polarity, or that power by which a magnetised 

 needle arrangus itself in reference to the magnetic poles 

 of the earth. But such considerations fail to assist us ; 

 and it will perhaps be better, after the introduction which 

 we have here given, to describe various polariscopes, by 

 means of which we may be able to investigate the sub- 

 ject experimentally, before proceeding to consider the 

 more intricate parts of our subject 



TO CONSTRUCT POLARISCOPES. 



IT is generally supposed that expensive apparatus is re- 

 quired for the purpose of studying the phenomena of 

 polarisation. This, like many other ideas of matters 

 required in the pursuit of science, is entirely erroneous. 

 It is true that extreme accuracy, such as is required in 

 exact research, can never be obtained except by employ- 

 ing instruments of the most careful construction. We 

 have been at some pains to find the cheapest and readiest 

 form of polariscope, of which any of our readers can avail 

 themselves; and after trying several experiments with 

 tliis view, we can recommend the following, which will, 

 at least, serve as an introductory arrangement to the 

 beauties of this branch of experimental science. In 

 every polariscope there are two essential parts ; namely, 

 the polariter and the analyser ; the former receives and 

 polarises the incident rays of light, and the latter presents 

 to the eye the polarised ray either by reflection or by 

 refraction. 



A polarising plate is easily made by coating a sheet of 

 window-glass, on one side, by means of lampblack 

 and size, or by holding it over the flame of a candle or 

 gas-light till completely blackened. We have found that 

 the flame of a "composite" candle answers best for this 

 purpose. The glass should be about eight inches square, 

 and should be placed on another piece of blackened glass ; 

 the two being separated by means of thin slips of card at 

 t !ii-ir edges, and the black surfaces should face each other 

 inside. This will prevent the black from being rubbed 

 from their surfaces whilst experimenting with it. We 

 may here remark, that so easy is it to obtain a polarising 

 plate, that we have often employed the slates of a roof 

 when wet by rain, or the horsehair covering of a parlour 

 cliair, for the purpose ; although tlte litter would scarcely 

 claim the dignity of a scientific instrument. To pre- 

 vent the glasses from slipping when in use, their edges 

 should be pasted together by means of narrow slips of 

 black paper. 



The analysing plate is made by piling up twelve pieces 

 of perfectly clean thin window or microscopic glass, which 

 must be closely packed together. They may be three 

 inches long, by about two inches wide ; and their edges 

 should be pasted by means of paper, <kc , so that they 

 may form a compact and immovable mass, as repre- 

 sented in the next figure. This alao excludes dust and 

 dirt. 



To use these arrangements for the purpose of polarising 



Ught, place the 

 glass plate at a window, 

 so that they may reflect 

 the imago of a white cloud ; 

 or place before the plate a 

 sheet of ground glass, so 

 that the light falling on the 

 jMilaris.-r may have a uni- 

 form white tint The analyser should then be held 

 in tho right hand, and the eye should view the plate 

 through it at a distance of about four feet. On looking 

 through the analyser in one direction, the blackened plate 

 will appear much of the same colour as if viewed by tho 

 naked eye ; but if tho analyser bo tunics! round through 

 a quarter of a circle (90), and the blackened plate be 

 still observed, it will be found, that nearly all the Ught 

 from its Hurface is lost. Thus, one of the phenomena of 

 polarUed light may IHJ noticed. If the analyser bo turned 

 ruim.l through half a circle, the light will again a]>, 

 if through three-quartan, or 270, it again vanishes, 



until the analyser has regained its original position 

 between the eye and the observer, when the light re- 

 flected and polarised by the black plate will again be ob- 

 served in its original intensity. If a piece of mica be 

 now interposed at a distance of a few inches from the 

 analyser, and between it and the polarising plate, it will 

 present, if placed at the proper angle, a beautiful ap- 

 pearance. The most magnificent colours will be seen; 

 and if the analysing plate be steadily turned round, to 

 the extent of 90, whilst the mica is held in the same 

 position, the original colours will entirely disappear, and 

 their complementary colours succeed them. * Instead of 

 a piece of mica, a thin plate of the yellow prussiate of 

 potass may be used ; aiid the effect is, if possible, still 

 more brilliant 



To enable the student to employ this simple piece of 

 apparatus successfully, we append an engraving, showing 

 as near as possible the relative position of the different 

 parts. We are aware that a little difficulty may be ex- 

 perienced, at first, in placing each at its proper angle ; but 

 after acquiring this, the student will find the arrange- 

 ment extremely convenient for many purposes. 

 Fig. 74. 



In Fig. 74, a represents the bundle of glass plates 

 forming tho analyser ; 6, the plate of mica under exami- 

 nation ; c, the polarising plate. Tho direction of the 

 polarised ray is from c, through d, the mica plate, the 

 analyser, and thus at hist it reaches the eye. In looking 

 through tho analyser, tho long edge of the glass plates 

 should bo held as to make such an angle with the nose 

 as that the observer may see the lino of light in a plane 

 passing about an inch from the lower und of the top 

 surface of the analysing glass; and which, if carried 

 through tho bundle of plates, would almost touch 

 the extremity of the under surface of the bundle, which 

 will thus be at an angle of about 57 with the horizon. 

 A most beautiful play of colours may be produced by 

 gently moving the plate of mica about in various direc- 

 tions. This, however, the student will soon discover. 

 By splitting pieces of mica up into plates of different 

 thickness, each piece may be made of such a size as to 

 reflect one colour only, and its complementary. A sheet 

 of mica, therefore, presents a great number of beautiful 

 appearances, owing to the various thicknesses of which 

 it is composed. One of the readiest ways of splitting 

 that sub.itanco is by making it red-hot ; a great number 

 of parallel plates are thus formed. 



Having described this apparatus, whic h may be made 

 for a few pence, and which will enable any one to 

 become acquainted with somo of the magnificent effects of 

 polarised light we proceed to point out other arrange- 

 ments answering the same purpose, but which are more 

 complete in their construction. 



The most simple of the kind that we have yet seen is 

 that constructed by Mr. Bestall, of Kemiington ; one of 

 which is exhibited at the South Kensington Museum. 

 Tho following engraving illustrates its arrangement ; 

 and the description wo give is in the inventor's own 

 wxxds. 



Description of Beslatt'a Polarising Apparatus. This 

 apparatus for showing tho beautiful phenomena of 

 See ante, p. 51. 



