928 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
hyperbolic brushes are in turn brought between the pair of vertical cross-wires, and 
observing the corresponding angular readings recorded by the goniometer circle and 
pair of verniers. After repeating the observations with light of the same wave-length 
once or twice, according to the definition of the brushes afforded by the particular 
section, the prism is rotated until light of the next desired wave-length is allowed to 
pass the exit slit, when the observations are repeated, and so on for as many wave¬ 
lengths as are desired. When once used to the a]’rangement a series of observations 
for the six wave-lengths mentioned at the close of the last section may be carried out 
in triplicate in less than half-an-hour. Employing the lime-light, the slits need only 
be opened so that the D lines would appear just coalesced if the entrance slit were 
illuminated by a sodium flame, and the spectrum were observed by placing the 
eye-piece in front of the exit slit. The illumination for D light is then as bright or 
brighter, even when the entrance slit is stopped down to ^ inch, than when the 
polariscope is placed directly in front of a good sodium flame, and the illumination 
for lithium light is vastly superior to that obtained by use of a lithium flame. 
Moreover, the illumination is quite evenly distributed over the field, and the inter¬ 
ference figures are wonderfully sharp. Results only slightly inferior are obtained by 
using the incandescent gas-light with a receiving slit of about twice the opening. 
After the conclusion of the measurements of the apparent angle (2E) in air, the 
glass cell containing n, colourless oil, or preferably the highly refractive liquid bromine 
derivative of naphthalene, a-monobromnaphthalene, is raised until the crystal is 
fully immersed in the liquid, and a similar series of measurements are made of the 
apparent acute angle (2 Ha) in the highly refractive liquid. The light may then be 
temporarily extinguished in the lamtern while the section is removed from the crystal 
holder, and the second section, perpendicular to the second median line, is placed 
there in its stead, and adjusted in white light by means of the goniometer lamp, 
whose by-pass has been left burning in order to save time in re-ignition of the lamp. 
The measurements of the apjaarent obtuse angle (2 Ho) of the optic axes in the same 
highly refractive liquid are then carried out for light of the same wave-lengths, and 
in a precisely similar manner as in the case of the first section. 
By thus carrying out the complete series of measurements with the two sections at 
one sitting, a process which need only occupy about an hour, all risk of any perceptible 
change in the refractive index of the liquid is avoided. The results thus obtained in 
so comparatively short a space of time, by the aid of the monochromatic light 
apparatus now described, enable the true value of the optic axial angle (2V a) and the 
mean refractive index yS, for six different wave-lengths, to be immediately calculated, 
and, if considered desirable, the value of these constants for any wave-length may be 
further expressed by embodying the results in a general formula of the type of that 
of Cauchy. Moreover, provided the sections are afforded by naturally-occurring 
largely developed faces, or are prepared by means of the apparatus described in the 
