914 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
(In crystals Ijelonging to the two systems of least symmetry) differences in the 
directions of stauroscopic extinction, observations with light of only these three wave¬ 
lengths are insufficient. Moreover, in the cases occasionally met with—such as the 
rhombic form of titanium dioxide known as brookite, the rhombic triple tartrate of 
sodium potassium and ammonium, and the monoclinic ethyl-triphenylpyrrholone 
described three years ago by the author,'* —in which the dispersion is so large that 
the axes for red light lie in a plane perpendicular to that which contains them when 
illuminated by blue light, observations with lithium, sodium, and thallium light 
are totally inadequate to enable us to follow the change which must occur as the 
wave-length of the light is altered, and, except by mere fortuity, afford no means 
whatever of observing the interesting point when the wave-length is such that 
the axes coincide in the centre of the field and the biaxial crystal simulates a 
uniaxial one. 
It is evident, therefore, that for the complete investigation of the optical properties 
of crystals, an arrangement for procuring monochromatic light must be adopted which 
will enable us to illuminate the field of the observing instrument with the whole of 
the spectrum colours in succession. A step towards supplying such a requirement 
has been made by Fuess, the well-known crystallographical optician of Berlin, in his 
larger axial angle goniometer. In front of the objective of the polariscope are placed 
a small prism and a collimating tube, arranged at such an angle to the polariscope 
that the light from a lamp passing through the slit of the collimator is dispersed by 
tlie prism into a spectrum, the whole of which is seen in the field on observing 
through the polariscope. The prism is capable of rotation, the amount of which is 
registered by a micrometer. It is intended that the readings of the micrometer shall 
be recorded for the coincidences of the vertical cross-wire of the polariscope with the 
principal lines of the solar spectrum, so that light of any particular wave-length may 
be brought into the centre of the field when using any artificial source of white light 
In practice, however, the author finds this arrangement unsatisfactory. The smallest 
amount of “ backlash ” in the working of the endless screw and wheel by which the 
rotation of the prism is effected introduces a considerable error in the reproduction of 
the setting for any solar line. But, even assuming the construction perfect at first 
and to remain so after use, the arrangement labours under the great inconvenience 
that the whole, or, when the second power is employed, almost the whole, of the 
spectrum is visible at once. Although it may be true that a fair approximation to 
the value of the o])tic axial angle for any wave-length may be obtained in cases where 
the dispersion of the axes for different colours is not considerable, by bringing light 
of that wave-length to the vertical cross-wire (or between the jDair of cross-wires) to 
which the hyperbolic brushes are also successively adjusted, still the rings and 
lemniscates surrounding the axes are distorted more or less according to the amount 
* ‘ Joui’ii. Chcm. Soc.,’ 1890, 733; ‘ Zeitscbrift fiij' Krystallograplue,’ XVIII., 563. 
