LANTERN POLARISING APPARATUS 345 



absolutely necessary to apply chamois leather, it should be a 

 piece of the softest, well beaten till free from dust, carefully 

 washed, and finally cleansed from all soap and grease in 

 alcohol. The polariser should always stand with the balsam- 

 joint perpendicular when the instrument is put away, else 

 the weight of the top piece of spar may produce ' thin film ' 

 colours in the balsam layer. 



Unfortunately Iceland spar has for some time ceased to 

 be imported, and been very scarce, so that it is doubtful if 

 material could at this moment be found for another large 

 prism, except the two specimen blocks in the British 

 Museum. This has confined opticians to smaller pieces. Mr. 

 Ahrens has constructed an ingenious prism of three pieces 

 as in fig. 191, the arrow showing the 

 optic axis. These form admirable ^, 

 polarisers, and only need spar of half \ 

 the usual length for the same sec- 

 tional area : but every one of several I 

 have tested fails as an analyser, the 

 edge of the wedge producing a percep- 

 tible though faint duplicate image, FIG. isi.- 

 and the latter being distorted. He 



has constructed also several polarisers of two inches square or 

 more, made in four separate prisms, which have been used 

 by Messrs. Newton & Co. for polariscopes and given satis- 

 faction. I have, I think, tested every prism so made ; and 

 the lines of junction being out of focus were not distinguish- 

 able, while the whole polariser was of less length than width. 

 Unfortunately spar has lately failed even for these prisms, so 

 far as projection instruments (other than microscopes) are 

 concerned, and this has caused attempts at constructing 

 ' direct ' polariscopes in other ways. The most obvious ex- 

 pedient is a bundle of glass plates in a tube as in fig. 188, 

 only larger, using the transmitted light ; but unless so many 

 plates are used as to absorb much light, it is difficult to get 



