918 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
the light of the wave-length for which the prism circle has been set shall always 
remain in the central line between the two jaws. The manner in which this object 
has been attained will be apparent from figs. 2, 8, and 4, which are reduced to about 
one-half the actual size. 
Fig. 2. Fig. 3. Fig. 4 
The principle lies in the adoption of an endless screw and wheel, in which latter two 
similar grooves are cut upon opposite sides of the centre, which, by means of sliding- 
pins connected with the jaws, bring about the desired equal and opposite movement 
of the latter. Fig. 2 is a front elevation of the slit-box, showing the endless screw a, 
which, for the sake of clearness, is not dotted and its nut-cap is omitted, and (dotted) 
wheel h. Fig. 3 represents a section of the box, showing the wheel h in its setting 
in the rigid framework of the box, the slide c, which carries the hard white metal 
bevelled jaws d, and the pin e, fixed in the slide, and whose motion is directed by 
the slots in the wheel. Fig. 4 is an elevation of the w'heel, showing the slots f which 
move the slides by means of the pins. The “ pitch ” of the slots is equal to one-half 
the total opening of which the slit is desired to be capable, which is adjusted so that 
the jaws may be sufficiently withdrawn to enable an unobstructed field of view to be 
obtained when the optical tube is used as a telescope for observing the solar or 
other spectrum. 
It is further provided that the white metal jaws may be removed altogether, in 
order that the single slit may be replaced by two or more whenever it is desired to 
employ composite light taken from definite parts of the spectrum. This is useful in 
order to be able to study the effect of such composite light upon the interference 
figures afforded by crystals whose dispersion of the optic axes for different colours is 
so great that the axes for red and blue light lie in planes at right angles to each other. 
The study of such figures in composite light is of assistance in appreciating the nature 
of the remarkable figures observed when white light is employed. It is for this pur¬ 
pose that the jaws themselves are not directly moved, by the wheel; they are held in 
close conta.ct with the slides c, which are directly moved, by being made to slide in a 
dove-tailed recess cut out of the latter, as shown in fig. 3, and when their knife-edges 
are brought just beyond the edges of the slides they are locked firmly to them by 
