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SUMMARY OF CURRENT RESEARCHES RELATING TO 



New Kinds of Glass of Increased Ultra-violet Transparency.* — 

 E. Zschimmer has renewed some earlier attempts to manufacture a glass 

 at Jena, which should be less opaque to ultra-violet radiation than is 

 usually the case. The only media at present known to answer the 

 purpose are quartz, fluorspar, and molten silica ; but these, excellent as 

 they are, are never likely to come much into ordinary use. The result 

 has been to discover a method, according to which various kinds of 

 optical glass of large size can be produced ; and these glasses are notably 

 more transparent for ultra-violet than the best glasses hitherto known. 

 Whilst with other glasses, 1 cm. thick, rays of wave-length 305 /a/a are 

 as good as completely absorbed, the new glass transmits about 50 p.c. 

 of the original intensity. 



The following table gives some of the optical constants of the new 

 glass (U.-V. glass). 



As for many purposes it is desirable that the visible spectrum should 

 be excluded, the inventor has endeavoured to meet this want, and he 

 has been so successful that the light as far as the blue is absorbed, 

 while ultra-violet as far as 280 /a/a is transmitted. This particular glass 

 therefore serves as a filter for photography : it is called " Violet, TJ.-V. 

 Glass" (No. 786'"). A U.-V. aplanat compared with an ordinary 

 apochromatic aplanat in an astrophotographical test showed 359 stars 

 in a Ursse minoris as against 264. Other tests were equally successful. 

 The glass is also produced as cover-glasses and window glass. A table 

 of photographs shows very clearly the superiority of U.-V. glass. 



Direct Micrometric Measurement of Fog Particles.! — C. Barus 

 succeeded in this object by means of a compound Microscope, magni- 

 fying about 100 diameters, provided with a filar ocular micrometer. 

 The objective and the whole lower part of the Microscope was sub- 

 merged in the condensation chamber and suspended from a wide rubber 

 cork through which it passed. All lenses below the cork were hermeti- 

 cally sealed with wax. The lower face of the objective was protected by 

 a glass shield, whose underside was covered by perforated wet blotting- 

 paper. Below this was a horizontal plate of thin microscopic glass 

 covered with a film of oil, and attached to a vertical brass rod by a 

 suitable clip. This rod passed through the cork and terminated in a 

 small lever by which the glass plate could be rotated and focussed. 



* ZeitBchr. Instrumentk., xxiii. (Dec. 1903) pp. 360-2 (1 pi. of photos, of spectra). 

 t Amer Jonrn. of Science, xvii (1904) pp. 160-70 (5 figs.) 



