es oer es a ea ae ee ee Cet ee Se 
W. Gibbs—Miscellaneous Optical Notices. 53 
orless solvent, whatever be its nature. By this means all angu- 
lar deviation and prismatic dispersion are avoided, as the color- 
ing matter does not sensibly change the refractive power of the 
liquid in which it is dissolved, and the incident beam passes 
through without change in direction. In my apparatus the 
wedges have acute angles of about 15°. This I find to be quite 
sufficient for most purposes, as it is easy to increase or diminish 
the quantity of substance dissolved. When it is wished to ex- 
amine the absorption produced by a definite thickness of liquid 
or by different thicknesses in succession, the double wedge is to 
be so placed that the slit of the spectroscope shall be parallel 
to the lines of intersection of the faces of the wedges. By mov- 
ing the wedge to one side or the other all thicknesses of liquid, 
from 0 to the maximum, obtainable with the apparatus used, 
may be successively examined. 
g 4 
On tests for the perfection and parallelism of plane surfaces of glass. 
When a plano-convex lens of long radius of curvature is 
placed upon a plane surface of glass and the system is illumina- 
ted by an obliquely incident beam of monochromatic light as, 
for example, by a sodium flame, the well known phenomenon of 
Newton’s rings is observed with remarkable distinctness and per- 
fection of definition. The symmetry of the rings will depend, 
in part, on the perfection of figure of the lens, in part on that of 
T- 
may of course be tested in succession, by moving the lens from 
point to point, and if necessary the rings may be observed with 
a Ler it in Sete es 
Prof. , of New York, has suggested for the observation 
TO 
scope. In his arrangement the lens and oe of glass are placed 
upon the stage of the microscope, the light from beneath be- 
ing cut off, and monochromatic light is then thrown down 
