930 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
one which j^reviously contained them, right up to their maximum separation for 
the last visible rays of violet. The exact position of the axes for any wave-length is 
at once obtained by setting the prism circle to the reading corresponding to the 
passage of light of that wave-length through the exit slit; and, in particular, the 
exact wave-length may be readily determined for the interesting case in which the 
two axes coincide in the centre of the field, when the rings and lemniscates become 
circles, and the biaxial crystal becomes apparently uniaxial. If it is desired to 
accurately measure the apparent angles for different wave-lengths the more delicate 
polariscopical goniometer is of course employed, the crystal being rotated 90° in its 
own plane after the measurements on one side of the central uniting point have been 
carried out in order to effect the remainder. For the purpose of merely studying or 
demonstrating the phenomena, however, the wide angle goniometer is employed, as 
the complete series of figures may then be observed in succession without moving the 
section, but simply by rotation of tlie prism of the monochromatic light apparatus. 
It is the author’s intention to communicate subsequently the results of a detailed 
investigation of a number of cases of crossed axial plane dispersion, illustrated by a 
series of photographic reproductions, which it is comparatively easy to produce with 
the aid of the apparatus now described in conjunction with a good camera. 
Mode of Production and Use of Composite Light. 
In the investigation of such cases of extreme optic axial dispersion it may be 
desired to supplement the measurement of the axial angle for different wave-lengths 
l^y observations of the interference-figures exlnbited in mixed light taken from any 
two or more known regions of the spectrum. For this purpose the jaws of the exit 
slit are removed and their place occupied by a diaphragm pierced by two or more slits 
so arranged as to permit of the exit of light vibrating with the desired wave-lengths. 
It is found preferable in practice to construct such diaphragms in the following simple 
and permanent manner, ready for immediate use at any time, rather than to employ 
an adjustable arrangement. The slide with three movable and adjustable shts 
employed by Abney, and so convenient for use with a fixed spectrum, is unsuitable 
when the spectrum is movable. 
There is provided with the instrument a slip of glass, finely ground upon one side, 
whose edges are bevelled and which is of the right size to be capable of sliding 
readily into the dove-tailed recess vacated by the slit jaws. When the slides c 
(fig. 3) are withdrawn, as far as possible, by rotation of the milled head in connection 
with the tangent-screw, the ground-glass surface may be employed to receive the 
solar spectrum or the spectra of metallic lines. Having decided what wave-lengths 
are to be permitted to escape through the two or three slits, the spectrum is brought 
into such a position by rotation of the prism that the lines corresponding to the two 
wave-lengths, or if three are required to the two outside wave-lengths, are about equi- 
