926 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
The wave-lengths mostly employed by the author in crystallographical investiga¬ 
tions are those corresponding to the red lithium line, the red hydrogen line C, the 
yellow pair of sodium lines D, the green thallium line, the greenish-blue hydrogen 
line F, and the bluish-violet hydrogen line G, supplemented by other well marked 
intermediate solar lines in cases of extreme optic axial dispersion. The angular 
difference between the circle readings for the lithium and G lines is 6° 10' in the 
author’s instrument, and as the readings can be made directly to minutes, and a 
deviation of one minute from the setting between the edges of the slit is readily 
perceived, it will be at once apparent that light of any wave-length can be produced 
with a very high degree of accuracy. 
Use of the Instrument in the Measurement of Optic Axial Angles. 
The whole arrangement for the measurement of optic axial angles—by the 
observation of the separation of the hyperbolic brushes of the interference figures 
atforded by a pair of sections perpendicular to the first and second median lines 
respectively in convergent light, and with nicols crossed at 45° from the horizontal 
Pig. 5. 
and vertical positions—is shown in fig. 5. The lantern and the axial angle gonio¬ 
meter are conveniently mounted upon firm box supports of rectangular shape and 
covered with dark red cloth like the dais of the monochromatic light apparatus, in 
order that they may be easily moved over the polished table into any required 
position. The block supports should carry levelling tables, each consisting of two 
mahogany boards, framed and with flush panels in order to inci’ease their rigidity; 
