MONOCHROMATIC LIGHT OF ANY DESIRED WAVE-LENGTH. 
927 
the lower of these boards is screwed down upon the top of the basal support, and 
carries four toe-plates in which rest four strong levelling screws working through 
nut-plates screwed to the upper board. The height of the blocks is so arranged that 
the optical axis of the light issuing from the condensers of the lantern, and that of the 
polariscope of the axial angle goniometer, may be adjusted by means of the levelling 
screws to exactly the same plane as that in which the axes of the optical tubes of the 
monochromatic light apparatus lie, that is to the level of the observer’s eye when 
seated. 
The axial angle goniometer may conveniently rest upon a circular base-board with 
grooved margin for the reception of a protective glass shade, rather than directly upon 
its levelling table; for the under surface of the base-board can be covered with cloth 
or felt, and the heavy instrument, together with the base-board, can then be readily 
moved about upon the polished levelling table, and the adjustment is facilitated. 
The crystal plate perpendicular to the first median line is first attached to the 
crystal holder, and adjusted by means of the adjusting and centering motions 
provided upon the goniometer. The section itself is either mounted upon a circular 
glass plate, as described in the preceding communication, or is suspended unmounted 
by means of a strip of thin glass, to which it is fixed at some point on its edge by 
means of a little marine glue or other cement which is not attacked by the highly 
refractive liquid to be employed, and which is held by the crystal holder. Sections 
prepared by use of the instrument described in the preceding memoir may always be 
suspended unmounted provided the specified time has been bestowed upon the final 
polishing, and the observations are then unaffected by slight errors due to the cover 
glass or want of parallelism in the cementing film. The adjustment is carried out in 
ordinary white light, so that the monochromatic light apparatus may not be un¬ 
necessarily used on these preliminaries. For this purpose the goniometer and its 
base-board are rotated through a right angle upon the levelling table, and the 
polariscope is illuminated by the goniometer lamp described in the foregoing pa])er. 
Having adjusted the section in white light so that the hyperbolic brushes and the 
rings and lemniscates are bisected by the horizontal cross-wire of the polariscopical 
eye-piece, the short tube carrying in its centre the more coarsely ground of the two 
diffusing screens is attached in front of the exit slit of the monochromatic light 
apparatus, so that the ground glass surface is distant about one and a half inches 
from the slit, the goniometer is rotated until its axis forms a continuation of that of 
the exit tube of the latter instrument, and moved up towards the ground glass screen 
until the end of the polarising tube enters the diffusing tube and all but touches the 
screen. The prism and the ends of the optical tubes are then covered by the dark 
box and the circle set to the reading recorded in the table for light of the wave¬ 
length to be first employed, usually that corresponding to the passage of red lithium 
light through the exit slit. The light is generated in the lantern, and the observa¬ 
tions are commenced by rotating the section in the usual manner so that the two 
