Chemistry and Physics. 491 



of which it treats. The publishers have presented the book to the 

 public in an attractive form. 



3. A Text Booh of Organic Chemistry ; by A. Bernthsek, 

 Ph.D. Translated by George M'Gowan, Ph.D. London, 1889. 

 (Blackie & Son.) This new text book merits high commenda- 

 tion. Its author was formerly Professor of Organic Chemistry 

 at Heidelberg and is at present director of the Baden Aniline 

 and Alkali Manufactory in Ludwigshafen-am-Rhein. He is well 

 and favorably known not only as a teacher but also for his re- 

 markable investigations in organic coloring matters. The trans- 

 lation has been faithfully made by Dr. M'Gowan of the Univer- 

 sity College of N. Wales, Bangor, and both gentlemen have con- 

 tributed to bring the book up to date. l. h. f. 



4. The spectrum of Hydrogen. — MM. L. Thomas and Ch. 

 Trepied obtain the lines Ha . H/3 by passing a stream of hydro- 

 gen gas between the carbon terminals of a voltaic arc. They 

 state that these lines can also be obtained by the use of steam 

 injected between the terminals. See Trowbridge and Sabine, 

 this Journal, Feb., 1889. — Comptes Pendus, p. 524. j. t. 



5. Spectrum of gases at loio temperature. — H. K. R. Koch 

 shows that the spectra of air, oxygen and hydrogen do not 

 change when the temperature of the environment in which the 

 electrical discharge takes place is —100° C. Any change there- 

 fore observable in the spectra of the northern light in polar 

 regions cannot be attributed to the temperature of the environ- 

 ment, but must be due to other causes. — Ann. der Physik und 

 Chemie, No. 10, 1889, pp. 213-216. J. T. 



6. JSfeio Photographic lens. — M. Ch. V. Zenger employs two 

 correction lenses of magnesium glass of the same focal length, one 

 concave and the other convex. The focal length of the system 

 is the same as that of the spherical mirror. The time of expo- 

 sure for stars of the same size is reduced to a third or a quarter. 

 — Comptes Rendus, No. 12, Sept. 10, 1889, p. 474. j. t. 



V. Pin-hole Photography. — A paper on this subject was read 

 at the late meeting of the British Association by Lord Rayleigh, 

 who showed that a simple aperture was as effective as the best 

 possible lens in forming an image, if the focal length (f) was 

 sufficiently great. Conversely, if /'be given the aperture may be 

 made so small that the use of the lens will give no advantage. 

 In some recent experiments the focal length was about nine feet 

 and the aperture T L of an inch. The resulting photographs 

 showed detail not materially less than that observable by the 

 ordinary eye. There would be no difficulty in working with an 

 aperture equal to the pupil of the eye, with a focal length of 66 

 feet, if extraneous light could be excluded. — Nature, Oct. 10, 

 1889, p. 584. j. T. 



8. Blue color of the sky. — M. Crova in collaboration with M. 

 Hondaille has conducted a series of observations at Mount 

 Ventoux on this subject. The apparatus employed was a modi- 

 fication of Crova's spectrophotometer (Annales de Chimie et de 



