MONOCHROMATIC LIGHT OF ANY DESIRED WAVE-LENGTH. 
923 
the particular optical instrument to be illuminated, and the screen chosen may be 
placed in any position in the carrying tube, best with the ground surface nearest the 
slit in order to avoid loss of light by reflection from the smooth surface. For certain 
work it is best to have it right at the end nearest the slit, so that by sliding the 
whole tube along the rod the screen may be brought close up to the slit frame ; 
while for other classes of work it is advantag-eous to remove it as far from the 
slit as possible by placing it at the other end of the tube and sliding the latter away 
from the slit as far as the length of the rod permits. For most purposes, however, it 
is best to place it in the centre of the carrying tube, when it is shaded on both sides 
from extraneous light, and the half of the carrying tube furthest from the slit serves 
as a dark box mto which the end of the observing instrument may be pushed until 
its objective almost touches the screen. 
The whole instrument is mounted upon a strong base-board, upon which it can be 
levelled by means of three strong levelling screws resting in toe plates. The base¬ 
board is conveniently covered with black velvet so that, with the aid of suitable 
folding screens constructed of strong cardboard and covered inside also with black 
velvet and outside with dark red cloth, the whole apparatus may be readily enclosed 
whenever desired (on account of imperfect transparency of the crystal under exami¬ 
nation) in a dark chamber and thus effectively shaded from stray light from the 
lantern. In ordinary cases, with good transparent crystals, it will be found sufficient 
to cover the prism and the ends of the optical tubes at which the lenses are placed 
with a dark box of the kind shown in fig. 1, also constructed of cardboard and 
covered inside with veh'et and outside with dark red cloth, and in which a small 
movable door is left through which the rotation of the prism can be effected. The 
base-board is in turn mounted upon a strong dais, of such a height above the table 
upon which the whole arrangement stands that the. plane of the axes of the optical 
tubes is raised to the level of the eye when the observer is seated. This dais is con¬ 
veniently covered with the same dark red cloth, which enables the base-board, whose 
under surface is smooth, to be easily moved over the dais and rotated 90° upon it, as 
will be subsequently shown to be desirable in order to be able to approach certain 
observing instruments sufficiently near to the slit, the dais otherwise being in the way 
of the support of the observing instrument. Moreover, if the table has a polished 
surface, the dais base-board and instrument can ])e readily moved en hloc to any 
required position. The instrument is so heavy, being so solidly constructed, that 
these apparently trivial aiTangements are of considerable moment. The base-board 
is grooved around its margin for the reception of a rectangular protective glass shade 
when the instrument is not in use. 
Determination of Circle Readings for the issue of Light of Definite Wave-lengths. 
The determination of the prism circle readings for the passage of ligFt of certain 
6 B 2 
