Physics and Chemistry. 347 
"gays are received upon a screen perpendicular to the axis o the cone and 
} uce a circular sector in which the maximum intensity of light cor- 
__ esponds to the rays, the plane of reflection of which is parallel to the 
: ~ of polarization. If we introduce a plate of quartz cut perpendicu- 
_ the branches of this cross will be replaced by a sort of fan exhibiting the 
different colors of the spectrum. j 
Measurements the details of which are not yet published have led 
Stefan to represent the rotations due to a plate of quartz 1™™, in thick- 
q a . 8 
hess by the formula p= 1007 
. in which the wave length 4 is expressed in thousandths of a millimeter. 
From this formula we should deduce the remarkable consequence that the 
@ Totation would be zero for rays of a wave length equal to 0:002186™™, 
pe consisted ing 
Sin’s rays by means of a concave mirror, then cutting off the light by 
invisible rays. Want of means appears to have prevented the execution 
r 
: ‘er. Tyndall employed an electric lamp of Duboseq and a linear thermo- 
ne pile, the spectrum being ‘ormed by lenses and prisms of rock salt. 
