MONOCHROMATIC LIGHT OF ANY DESIRED WAVE-LENGTH. 
935 
order that the crystal may be well seen when using the telescope as a microscope by 
rotating the movable lens into position in front of the object-glass. But for the 
accurate adjustment of the images of the slit, reflected by the faces of the prism, to 
the cross-wire of the telescope when measuring the angle of the prism, just as in 
making ordinary goniometrical measurements, a dark background is required. In 
order to supply either background as desired, and with the minimum of trouble, the 
little arrangement shown behind the goniometer in the illustration serves admirably. 
It consists of a strong brass pillar screwed into a bevelled foot-plate which is capable 
of sliding in a dove-tailed groove in the heavy metal base, so that the pillar may 
readily be brought opposite the telescope when this cannot be done by movement of 
the whole without bringing the base partly over the edge of the supporting box. 
The pillar is pierced by an axle at about a third of its height, to the front end of 
which is attached an arm which carries the dark background in the form of an ebonite 
sector. The axle is easily moved by a lever handle attached to the axle at the back 
of the pillar. The amount of rotation is limited by cutting a piece out of the axle 
nut into which the arm is screwed, and fixing a pin into the bearing, so that the axle 
can only be moved round a little more than 45° from the position in which the arm is 
vertical, when its motion is arrested by the stop-pin. The white background is 
formed by a similar sector of white xylonite fixed to the pillar. When the movable 
arm is vertical the ebonite sector completely covers the white background ; when the 
lever is touched the ebonite sector moves over to the left, and a large portion of the 
white xylonite sector is exposed. In order to screen the crystal and the objectives 
of the collimator and telescope from any overhead light, a thin metallic canopy, 
darkened underneath, is suspended over the goniometer from a bent brass rod resting 
loosely in a socket drilled into the top of the pillar which carries the backgrounds, so 
that it may be rotated out of the way while reading the verniers. 
Ample light for reading the verniers may be obtained from the goniometer lamp by 
temporarily opening widely the door of the aperture. 
Having measured the angle of the prism, the direct reading of the slit of the 
collimator may be taken at once while the goniometer lamp with protective cylinder 
is in position, and the position of minimum deviation of the spectrum produced by 
the crystal prism also found, so that time may be saved when using the monochro¬ 
matic light arrangement. The Websky slit employed in the preliminary gonio¬ 
metrical observations is retained for the measurement of minimum deviation. This 
slit is formed by using as jaws the adjacent portions of two circular discs, whose 
circumferences almost touch, the aperture thus produced combining the advantage of 
a narrow middle portion which can be accurately adjusted to a cross-wire, with 
broader ends which pass so much more light. 
It will be observed in fig. 7 that the dais of the monochromatic light apparatus 
is rotated under the base-board for a right angle ; this is advisable in order that the 
large base of the goniometer may be pushed for a little distance under the base-board, 
