40 



unequal tension, caused by the contraction of the surface, and the interior 

 trying to shrink into a smaller compass, but being prevented by the hard 

 jacket outside. Taking one of these as a standard, he was enabled to 

 determine easily the condition of the piece of mica. With the crossed 

 Nicol's Prisms, there would perhaps be found a black cross bisecting the 

 white square, in consequence of the glass being in a state of tension — 

 now suppose a piece of mica, blue of the second series, placed over the 

 glass gave the colours green and red, the direction of tension of the mica 

 would be the same as that of the green parts of the glass. Having 

 determined one film and marked it, it was very easy to determine others. 



There was another fact that was useful, not so much for instruction as 

 for the beauty of some of its effects. Under ordinary circumstances the 

 colours simply passed from the red into green through the neutral zone. If 

 a piece of mica were used which had only the thickness which corresponded 

 with the quarter of the space between the first purple and the commence, 

 ment of pure white [as shown on a diagram], then it would be found that 

 by this so-called 5 film, when placed over any of the doubly refracting objects 

 with its principal section corresponding with, or at right angles to, the plane 

 of polarization, the light is so retarded that, instead of plane polarized 

 light, there would be produced circularly polarized light. The advantage 

 was this, that where with a simple selenite no colour was obtained, with the 

 i film the colours would change on turning from indigo blue, through yellow, 

 orange, red, and purple, to green, which of course greatly enhanced the 

 beauty of many objects when viewed with a £ film. 



The question then arose, What thickness of film was most useful ? He 

 had not the slightest doubt it was what was known as the blue of the third 

 series. This was the most sensitive of all, simply because this blue was a 

 very narrow belt ; on one side close against it was a very bright red, and on 

 the other side a brilliant emerald green. A very thin film will at once change 

 this blue to brilliant green or brilliant red, either contrasting vividly w T ith 

 the pale blue ground. 



If the deep blue of the second series were used, it would not give half the 

 effect, because it was so broad. 



In one of the table polariscopes he would place a couple of films touch- 

 ing each other; over that he would put a blue film of the same thickness 

 as that combined with the orange. In one position this makes the blue 

 black; in the other the blue becomes red, and the orange becomes green, 

 according to the position of the film. Take off the blue film and rotate the 

 analyser, the orange becomes blue and the blue orange. If, however, the ? 

 film be used, instead of each passing through a neutral point to its compli- 

 mentary colour, it would pass through all the series of colours. 



Norremberg's Doubler was a simple arrangement for passing the rays 

 twice through the film, which was the same as if we had the power of 

 practically doubling the thickness of the film. This arrangement was par. 

 ticularly useful for testing the thickness of films. 



The eye was very treacherous as regards colours, and could not always be 

 trusted, but by the above plan the accuracy of one's judgment can be tested. 



