W. LeConte Stevens—Recent Progress in Optics. 383 
interesting result that the transmission curve for 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 
radiation 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 independence 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,* who demonstrated that the retarda- 
tion of the ray is proportional to the load. An extended 
series of such experiments has been lately made in this coun- 
try by Marston,t who, besides confirming Wertheim’s conclu- 
sion, 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 prin- 
cipal 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,t 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§ has ingeniously applied Faraday’s principle of 
electro-magnetic rotation of the plane of polarization in carbon 
disulphide 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 Nicol 
prisms. The combination becomes a chronograph with an 
index as free from inertia as the beam reflected from a galva- 
nometer mirror. The same instrument has been applied to 
measurement of the velocity of projectiles,| with results of 
exceeding interest to the student of gunnery. 
* Comptes Rendus, xxxii, p. 289, 1851. 
+ Physical Review, September-October, 1893, p. 127. 
t Journal de Physique, III, iii, p. 452, 1894. 
§ Transactions of the American Institute of Electrical Engineers, October, 1894, 
p. 591. 
| Journal of the U.S. Artillery, vol. iv, No. 3, p. 409, July 1895. 
