RECENT PROGRESS IN OPTICS. 131 



the ordinary ray is wholly independent of that for the extraordinary, 

 the absorption being in general much greater for the former. Several 

 sharp absorption bands are found for each ray. For radiations whose 

 wave-length exceeds 3.2 microns, the absorption of the ordinary ray is 

 almost complete, so that calcite behaves for such radiation just as 

 tourmaline does for the rays of the visible spectrum. The independ- 

 ence of the two transmission curves is found to exist also for quartz 

 and tourmaline, these curves for the latter crossing each other twice 

 in the infra-red region. 



The application of polarized light to the investigation of internal 

 stress in transparent media was made more than forty years ago by 

 Wertheim (Comptes renclus, 32, p. 289, 1851), who demonstrated that 

 the retardation of the rays is proportional to the load. An extended 

 series of such experiments has been lately made in America by Marston 

 (Physical Eeview, September, October, p. 127, 1893), who, besides con- 

 firming Wertheim's conclusion, shows that, "for small strains at least, 

 the colors seen in a strained glass body, when polarized light is passed 

 through it in a direction parallel to one of the axes of strain, are 

 measured by the algebraic difference of the intensities of those two 

 principal strains whose directions are perpendicular to the direction of 

 the polarized light." 



A new substance with double rotatory power, like quartz, has been 

 discovered by Wyrouboff (Journal de Physique (3), 3, 452, 1894), the 

 neutral anhydrous tartrate of rubidium, which is unique in one respect. 

 The rotatory power of the substance in the crystalline state becomes 

 reversed in solution. This wholly new phenomenon introduces some 

 perplexity in connection with certain molecular theories that have been 

 formulated to account for double rotatory power. 



Crehore (Transactions of the American Institute of Electrical Engi- 

 neers, October, 1894, p. 91) has ingeniously applied Faraday's principle 

 of electro-magnetic rotation of the plane of polarization in carbon 

 bisulphide to the photographing of alternate current curves. Every 

 variation in the magnetic field causes variation in the amount of light 

 transmitted through a pair of crossed ISTicol prisms. The combination 

 becomes a chronograph with an index as free from inertia as the beam 

 reflected from a galvanometer mirror. The same instrument has been 

 applied to measurement of the velocity of projectiles (Journal of the 

 United States Artillery, p. 409, July, 1895), with results of exceeding- 

 interest to the student of gunnery. 



PHYSIOLOGICAL OPTICS. 



The temptation to dilate upon recent progress in physiological optics 

 has to be resisted. The revision of Helmholtz's great book on this 

 subject was interrupted by the death of the distinguished author, but 

 the last part is now approaching completion under the care of his 

 pupil, Arthur Konig, who, in conjunction with Diederici, has done 



