Geology and Mineralogy. 327 



are completed, the colors of the spectrum become visible. The 

 theory of the experiment is very simple. The incident light 

 interferes with the light reflected by the mercury; consequently 

 a series of fringes are formed in the sensitive film, and silver is 

 deposited at places of maximum luminosity of these fringes. 

 The thickness of the films is divided according to the deposits of 

 silver into laminae whose thicknesses are equal to the interval 

 separating two maxima of light in the fringes — that is, half the 

 wave length of the incident light. These laminae of metallic 

 silver formed at regular distances from the surface of the film, 

 give rise to the colors seen when the plate is developed and dried. 

 — Comptus Benches, Feb. 2, Nature. Feb. 12, 1891, p. 360. j. t. 



9. Pin-hole Photography . — Lord Rayleigh, in a valuable paper 

 on this subject, discusses the subject of the definition obtain- 

 able by apertures of various diameters; and shows that a pin 

 hole may replace a lens under certain conditions. To obtain 

 however the definition of a lens of four inches in aperture a focal 

 distance of five miles would be necessary. With an aperture of 

 "07 of an inch and a focal distance of seven feet a photograph 8X 

 10 inches was taken of a group of dark trees which gave as much 

 detail as a lens covering the the same plate. Lord Rayleigh 

 finds the following relations, 2r 2 = fA in which r is radius of 

 aperture,/" is focal distance and A is wave length of light. — Phil. 

 Mag., Feb. 1891. J. t. 



10. Lectures on the Electromagnet ; by Silvanus P. Thomp- 

 son. 287 pp. 12mo. New York, 1891. Authorized American 

 edition. (W. J. Johnston Co.). — Modern technical electricity 

 may be said to rest in large measure upon the development of 

 the electromagnet, and hence the importance of this excellent 

 treatise from the able pen of Prof. Thompson. The volume is a 

 small one, and the subject large, but the author has found it pos- 

 sible to present the matter with much fullness and completeness. 

 The history of the electromagnet is given with an account of the 

 early experiments of Sturgeon and Henry, and this is followed by 

 a description of the principles involved in their construction, the 

 various forms applicable to different purposes and their use in 

 the many forms of mechanism in which they form an essential 

 part. The matter was presented in four lectures delivered a little 

 more than a year since before the Society of Arts in London, and 

 hence the form of presentation gains that directness which exists 

 when the speaker comes into immediate contact with his audience. 



II. Geology and Mineralogy. 



1. Lake Bonneville; by Grove Karl Gilbert. Monographs 

 of the U. S. Geological Survey, volume i, 438 pp. 4to, with 51 

 plates and a map. Washington, 1890. — Mr. Gilbert's history of 

 Lake Bonneville — the largest of the Quaternary lakes of the 

 Great Basin, of whose area it covered about one-fourth, em- 

 bracing Great Salt Lake of to-day — is an admirable exposition 



