920 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
of crystals of imperfect transparency. For use witli clear sections and for all cases 
where the dispersion amounts to or exceeds the limit just specified, and also for use 
in all determinations of refractive index by means of prisms, a series of four stops, 
perforated by circular apertures of |-inch, ^-inch, f-inch, and ^-inch diameter respec¬ 
tively, are provided by which the width of the slit may be suitably diminished. The 
stop most frequently employed by the author is the one of |-inch diameter, wFich 
affords spectral lines which are apparently perfectly straight. This definite mode of 
reducing the width of the slit is found more convenient than by use of the usual 
>-shaped arrangement, and it is more satisfactory to have the ends perpendicular to the 
length of the opening. The stops are fitted with a light spring at one side to keep them 
in position in the rectangular recess in front of the slit. Two of them are shown in 
fig. 1 lying on the base-board. The illumination of the field of the observing instru¬ 
ment, when the slits are nearly closed and the quarter-inch stop is placed in front of 
the receiving slit, is still sufficiently good, when the lime-light is the source of light, 
to enable excellent measurements of axial angles or refractive indices to be carried 
out with F light, and when the sections or prisms are clear with G light. If it is 
inconvenient to employ the lime-light, excellent measurements may still be obtained 
as far as F light by substituting for it in the lantern the improved form of incan¬ 
descent gas-light burner, as described in the preceding communication, and slightly 
increasing the opening of the receiving slit. 
The slit frame at the end of each optical tube terminates in a slightly projecting 
annulus, m in fig. 2, of one and-a-half inch diameter, carrying on its outer surface a 
screw-thread upon which can be screwed the small eye-piece tube, which serves as a 
carrier for either of three eye-pieces, magnifying respectively two,four, and six diameters. 
The tube and its three eye-pieces are shown slightly to the right of the centre of 
the base-board in fig. 1. The eye-pieces are constructed to focus the closely approxi¬ 
mated jaws of the exit slit immediately in front of them, so that when the spectral 
lines are focussed by means of the rack and pinion movement which adjusts the 
distance between the lens combination and the slit, the knife-edges of the slit jaws are 
likewise in focus, and serve all the purposes of a parallel pair of vertical cross-wires 
between which the spectral lines may be adjusted by suitable rotation of the prism. 
Each optical tube is ca])able of independent rotation round the axis of the instru¬ 
ment, by means of the counterpoised arms. Each may be fixed in any position, by 
means of clamj^ing-screws, to the lower circle which carries the vernier and which is 
rigidly fixed to the central pillar of the strong stand, and whose plane is accurately 
perpendicular to the axis of rotation of the optical tubes and of the prism. The prism 
is carried upon a rotating table parallel to the lower circle, and which is graduated for 
180°; the graduations read directly to half-degrees, and, wdth the aid of the vernier, 
to single minutes. This rotating circle may be fixed for any reading by means of the 
clamping arrangement seen in front of the prism in fig. 1. A fine adjustment is pro¬ 
vided for the circle, and it is made readily detachable, so that it may be arranged at 
