MONOCHROMATIC LIGHT OF ANT DESIRED WAVE-LENGTH. 
921 
any convenient position on the limb, or may be removed altogether if not required. 
It is shown in position in fig. 7. It is constructed in two parts. A double elbow- 
piece, fitting closely to the circle-plate, and capable of being tightly clamped to it by 
means of two milled-headed screws passing through the upper plate of the piece, 
carries an outwardly projecting arm ; the latter is pressed between the ends of a long 
milled-headed screw of fine thread and a spring piston, similar to those employed in 
the fine adjustment of the circle of the instrument described in the preceding com¬ 
munication. The tangent-screw and piston are carried by a second elbow-piece 
enveloping a segment of the limb of the lower fixed circle ; the upper plate of this 
elbow-piece is sufficiently short (radially) to permit the upper elbow-piece to move 
past it without touching ; but the lower plate is longer, in order to afford a rigid grip, 
and carries the two clamping-screws, by which it may be fixed from underneath to 
the circle. The graduated surface of the movable circle is protected from the upper 
clamping-screws by means of a thin intermediate plate of hard white metal, lined next 
to the graduated surface with chamois leather. The distance between the ends of the 
nut of the tangent-screw and the cylinder of the piston is sufficiently great to enable 
the projecting arm, and with it the circle, to be moved by rotation of the tangent- 
screw through a little more than 7° of arc, sufficient to enable the whole spectrum, 
from A to a little beyond G, to be brought past the exit slit. The prism is firmly fixed 
to the rotating circle by means of an angle bracket and screw, which latter is prevented 
from injuring the top of the prism by causing the pressure to be applied to a slightly 
convex hard white metal plate, shaped like a three-rayed star, the three terminations 
of which rest upon the top of the prism ; the centre of the plate is perforated with a 
small hole, into which the rounded end of the screw fits without being able to pass 
through it. The lower portion of the strengthening rib of the angle bracket may be 
conveniently utilised as a handle, with which to effect the rotation of the prism and 
circle, whenever the fine adjustment is not in use. 
The 60“ prism is larger than usual, having sides of four-and-a-half by two-and-a-half 
inches, in order to be able to utilise as much of the light from the two-inch collimating 
lens combination as possible. The heavy flint-glass, which was supplied by Messrs. 
Chance, possesses as high a dispersive power as it was possible to obtain without 
introducing colour, in order that the dispersion shall not suffer much by the use of 
only one prism. There is a limit to the dispersion which can be employed, for if it is 
excessive, as by use of some of the very dense glasses now available, it is found that 
the whole of the spectrum cannot be brought to pass the exit slit by rotation of the 
prism without serious loss of light by reflection from the receiving surface, owing to 
the large angle through which the prism requires to be rotated. The essentials of the 
prism are, therefore, that it shall be free from colour in order that it may fully transmit 
the blue end of the spectrum, and that it shall possess the highest possible dispersion 
which will still enable the whole of the spectrum, from A to H', to be brought between 
the nearly closed jaws of the exit slit by rotation of the prism without materially 
MDCCCXCIV.—A. 6 B 
