39 



to rest. In other words, there would be a polarized object presenting a 

 green colour. 



To ascertain whether any object would polarize or not, it was necessary to 

 turn it round in all positions ; so the polariscope must be capable of rotation, 

 or the objpct must be capable of rotation on it. Sometimes it was more con* 

 venient to rotate the polarizer, sometimes the object. The analyser should 

 also have a rotating motion. 



He then explained by means of a diagram in which the bands of colours 

 given by different thicknesses of films were shown in their proper position, 

 that the effect was precisely the same whether there was a minimum of 

 tension or a minimum of thickness in a given film, and traced the changes 

 of oolour given by the different thickness of films of mica. 



In practice, it was often found more convenient, instead of trying to 

 split a film to a particular thickness, to use two or more films of mica or 

 selenite of a uniform thickness. 



Taking a glass disc on which were fixed two small semicircles of miGa 

 of different colours, he placed over them another larger circular film with a 

 hole in the centre, and, placing the two plates on the table polariscope, 

 the effeot of the added film was seen at a glance. 



Through the central aperture, the single films could be seen ; further out, 

 the result of the added film j and, beyond the smaller films, the colour of 

 the added film. The changes effected by rotating the films were shown 

 with the greatest clearness and simplicity. 



Then as to the various thicknesses of films which were best to use in the 

 microscope, If it was desired to produce the more beautiful effects, it was 

 simply a matter of taste ; some preferred the most brilliant colour, with 

 dashes of black, giving more vivid features such as so many people admired. 

 A Spanish lady would prefer scarlet or orange with a black shawl, and she 

 would show her good taste. 



There were others who would prefer softer, gentler tints of pink, and so 

 on, and it could not be said they were wrong. 



Those who wanted vivid colours of the first type should view the object 

 without selenite; with selenite the black of the first case is replaced by the 

 colour of the selenite used. 



What was actually learned by using the selenite film, with the least possible 

 trouble, was not only whether a body was in a state of tension or not, but 

 what was the direction of the teusion, whether pulled, strained, or squeezed. 

 This was done in a second, because if it were known what colour an object 

 presented with any given film of selenite whose direction of tension was 

 known, say blue, and the object previously blue appeared black when 

 examined without the selenite, it was perfectly clear that the tension 

 was at right angles to that of the film of selenite. 



But how can the direction of the tension in the film of mica or selenite be 

 determined ? Many years ago he adopted the following plan: — He took 

 small squares of glass, placed them in an ordinary clay pipe, put them into a 

 clear fire until of a nearly red heat, and then cooled them at an open window. 

 A good many of them cracked, but he obtained a number in a state of 



