328 SCIENTIFIC AMUSEMENTS. 



of the different directions of the path pursued by the light whilst 

 passing through the glass as compared with that whilst passing 

 through the air (Expt. 347); so will a plate of certain kinds 

 of crystallised matter, when cut in a particular way relatively 

 to the planes bounding the crystal, refract the light in two ways, 

 part passing along one path in the solid plate and part along 



another, so that two rays of light 

 finally emerge, the directions of which 

 are not coincident. The effect of 

 this is that a double image is per- 

 ceived through the plate. Thus fig. 



1^ indicates the effect of looking at 



Fig. 174. Double Refraction.' ^ e letters AB through a plate of 

 " Iceland spar, a natural variety ot 



crystallised carbonate of calcium ; the letters appear in duplicate, 

 owing to double refraction taking place. 



With all doubly refracting crystals there is one (and sometimes 

 more than one) position fin which the crystal may be held, such 

 that light passing through in a particular direction does not suffer 

 double refraction ; this position can easily be ascertained by trial in 

 the case of Iceland spar. Of the two paths pursued by a ray of 

 light when doubly refracted, one obeys the law of sines (Expt. 333), 

 and is spoken of as the ordinary ray, whilst the other does not 

 obey that law, and is termed the extraordinary ray. With 

 Iceland spar, ruby, sapphire, ferrocyanide of potassium, and other 

 substances, the former is the one more bent out of the original path 

 than the other ; with some other substances, such as ice and quartz, 

 the opposite is the case. In consequence, if a dot of ink on a 

 sheet of white paper be looked at through a piece of Iceland spar 

 two images will be seen, of which one will appear slightly nearer 

 the eye than the other ; this is the ordinary image formed by the 

 ordinary rays, which, being most refracted, will cause the greatest 

 apparent raising of the image. If the dot be viewed in a direction 

 at right angles to the paper and the crystal be turned round with 

 the face still on the paper, the ordinary image will appear stationary, 

 whilst the other will apparently revolve round it, the line joining 

 the two being always in the direction of the shorter diagonal of 

 the face of the crystal ; if the edges are of equal length the face 

 has the shape of a rhomb. The two rays also differ in certain 

 other respects, the study of which is intimately connected with 

 the phenomena termed polarisation of light, a branch into which 

 the limits of the present work render it impossible to enter. 



Expt. 359. Production of Colours by thin Films Iride- 

 scence. When a soap bubble is blown, as in Expts. 299 et seq., 



