540 SUMMARY OF CURRENT RESEARCHES RELATING TO 



thus permitting the identification of certain minerals and gems; (2) 

 interference spectra, thus assisting in determining the cause of lustre 

 and iridescence. In the first class the method is very effective in 

 dealing with minerals of the didymium and zircon groups, garnets, 

 rubies, spinels, emeralds. The spectra are frequently so characteristic 

 as to give infallible means of identification. For reference all spectra 

 should be recorded diagrammatically on paper ruled with lines corre- 

 sponding to the scale showing width of band : it should also be noted 

 whether edges are sharp or misty, and whether the darkest part of band 

 is central or eccentric. 



In dealing with the second class it should be remembered that lustre 

 and iridescence include such causes as simple reflection, refraction, and 

 dispersion, scattering of light from microscopic particles, polarization, 

 and diffraction. The most potent cause, and that to which practically 

 all iridescence is due, is the interference produced by reflection from 

 thin films, and this can be advantageously studied with the micro- 

 spectroscope. Such interference colours generally show dark bands in 

 the spectrum, one in the lower order colours produced by thin films, 

 and two or more as the films become thicker so that additional wave- . 

 lengths interfere. For comparison, records should be made of the 

 spectra of all the brighter colours, which can be done by observing them 

 in the " Newton's rings " produced between two surfaces of glass or by 

 blowing a bubble of melted glass until it bursts, when the thin edges 

 will answer the same purpose. The author has examined the natural 

 iridescent surfaces of many minerals, butterfly scales, beetle scales, 

 iridescent birds 1 feathers, opals, the chatoyance of cat's-eye germs, and 

 the lustre of pearls. His experiments seem to show that the explana- 

 tions usually given of certain phenomena, e.g. of butterfly scales, stand 

 in need of revision. Thus the particularly brilliant blue spots on the 

 wings of Papilio paris are apparently due to a film of air of about ' 58 /a 

 in thickness. Again, the lustre of pearls is not due to minute corruga- 

 tions, but to repeated parallel laminas separated by extremely thin films. 



t4) Photomicrography. 



Leitz' Small Photomicrographic Apparatus.* — This apparatus, 

 numbered IV. in the maker's catalogue, is shown in fig. 63. It has a 

 camera extension of 25 cm., and is designed for use in the vertical 

 position only. It consists essentially of a rectangular base-plate of iron, 

 covered with cloth, on one side of which a vertical steel rod is mounted 

 in a massive socket. This rod carries the camera, which by means of 

 the two clamping fittings (shown in the illustration) can be fixed at any 

 required height. When so fixed the camera can be rotated for the 

 final adjustment of the specimen on the stage by direct observation ; 

 the return of the camera to exact alignment with the optic axis of the 

 .Microscope is ensured by the provision of a spring-catch on the camera 

 fitting, the catch engaging with a groove in the steel rod. The length 

 of the camera is adjusted by sliding the rod, carrying the front up or 

 down in the grooved fitting in which it is monnted. A light-tight 



• Leitz' Catalogue, 43 G, Photomicrographic Apparatus, p. 18. 



