` 
466 The American Naturalist. [May, 
MINERALOGY." 
Recent Books. 
Fletcher. The Optical Indicatrix and the Transmission of 
Light in Crystals.’—This treatise is an important one for mineral- 
ogical students because of its simple form and style and its easy mathe- 
matical demonstrations. It is Fletcher’s opinion that, since Fresnel’s 
hypothesis that the medium for transmission of light—the ether—is in- 
compressible, has been shown to be untenable, it should be abandoned 
by teachers. The newer theory of Neumann and MacCullagh that the 
ether is compressible, he supports, but wisely advises that since we may 
be dealing with only an approximate mechanical analogy, it is best to 
make use of terms which do not commit one to either of the hypothe- 
ses. The surface of elasticity he proposes to call the optical indicatriz. 
The wave surface of Fresnel he prefers to call the ray surface, and ray 
front is substituted for wave front. The plane passing through the 
ray and the normal to the plane of polarization, he designates simply 
as the transverse plane. The optic axes of biaxial crystals are called 
bi-normals, and the “ secondary optic axes,” or directions of common 
ray velocity, he calls bi-radials. The author shows that Fresnel did 
not, as supposed, arrive at the wave surface as a deduction from his 
theory of the incompressible elastic ether, but by a simple generaliza- 
tion before his theory of the ether was developed. Chapter II, of 
Fletcher’s work, is devoted to the evolution of the optical indicatrix, 
and begins by remarkably simple statements of the accepted views con- 
cerning the nature of light, its transmission in isotropic media, reflec- 
tion, refraction, polarization, etc., with the facts on which they are 
e ray surface and indicatrix are then developed from em- 
pirical data. It is unfortunate that the printing of the book should 
have been so badly done. It has been translated into German by 
Ambronn and König’ - 
1 Edited by Dr. Wm. H. Hobbs, University of Wisconsin, Madison, Wis. 
? The Optical Indicatrix and the Transmission of Light in Crystals, By L. 
Fletcher, M. A., F. R.S. Pp. xii and 112. Henry Frowde, London, 1892. 
guaman Dói the Mineralogical Magazine). 
Die Optische Indicatrix, Eine geometrische Darstellung der age 
in den Krystallen, von L. Fletcher, uebersetzt von H. Ambronn u. W. Köni 
Pp. ix and 69. Barth, Leipzig, 1893. Price, M. 3. 
