MONOCHROMATIC LIGHT OF ANY DESIRED WAVE-LENGTH 
929 
preceding communication, the accuracy of the values furnished is of the very highest 
order. 
Use of the Instrument loith the wide-angle Polariscope, with particidar reference to 
the study of Crossed Axial Plane Dispersion. 
It is frequently desired to employ a wide-angle polariscopical goniometer, such as 
the well-known instrument forming part of the universal apparatus constructed by 
Fuess, of Berlin, at the instance of Groth. The aperture of the polariscope of this 
instrument is considerably larger than that of the more accurate instrument reading 
to thirty minutes of arc represented in fig. 5, and which is employed, as previously 
described, in the actual measurement of optic axial angles. The convergent system 
of lenses is also so powerful, consisting of several lenses of very short focus, that a 
very wide angle is included in the field of view, so that the rings and leinniscates 
surrounding both optic axes of most biaxial crystals are visible through a section per¬ 
pendicular to the first median line. This instrument is. therefore, very convenient for 
studying the nature of the interfereuce-figures, especially in cases of strongly-marked 
dispersion of the optic axes for different colours. The optical tube which carries the 
analysing nicol is provided, in addition to cross-wires, with an etched scale, which 
enables a rough estimation of the separation of the axes to be effected without 
rotating the section, and thus permits the convergent lenses of the two optical tubes 
carrying the polarising and analysing nicols respectively to be brought ahnost in con¬ 
tact with the two surfaces of the crystal plate, when the full aperture of the instru¬ 
ment is utilised. When desired, however, the tubes may be withdrawn sufliciently 
apart to permit of the rotation of the crystal plate, and of measurement of the separa¬ 
tion of the axes by means of the circle and vernier, which read to minutes, hut, of 
course, a smaller field and angle of view is presented. 
Even the comparatively large field of this instrument, whose objective has an 
aperture of 1-| inches, is fully and evenly illuminated ujdod placing it in front of the 
coarser ground-glass screen of the monochromatic light apparatus. The diftiising-tube 
is sufficiently wide to admit the end of the polarising-tube, so that the latter may be 
brought close up to the screen. It is not necessary to use more than the f-inch slit 
so that the monochromatism can be made as perfect as when using the more delicate 
instrument with smaller objective. 
The investigation of cases of such extreme dispersion as to result in the optic axes 
for red and blue lying in different planes, may be very beautifully carried out with 
the aid of the instrument for producing pure monochromatic light now described. 
The whole phenomena may be‘ traced with the utmost precision, from the extreme 
separation of the axes for the first rajs of red, through the gradual approach of the 
axes with diminishing wave-length, until they unite in the centre of the field ; and 
subsequently as they re-diverge along the diameter of the field peiqjendicular to the 
MDCCCXCIV.—A. 6 C 
