940 
]\m. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
that the end of the tid^e which carries the polarising nicol almost touches the gTound- 
glass diffusing screen, carried as usual in the tube mounted in front of the exit slit, 
and to make the axis of the microscope a continuation of that of the exit tube of the 
monochromatic light apparatus. 
In order to nse horizontally the stauroscope forming part of the universal apparatus 
snpjDlied by Foess, a very simple addition in the form of a supporting stand of wood 
is all that is required. The stauroscope is supported horizontally upon the base-board 
by utilising the metal foot, upon which the instrument usually stands, as one support, 
and a similar one of hard wood firmly fixed to the base-board as support for the 
other end of the rod of triangular section along which the optical tubes slide, and from 
which they now depend. The opening between the two toes of the metal foot is ample 
to permit of the full aperture of the instrument being employed, and the toes are 
prevented from slipping off the base-board by means of a low protective rib of similar 
wood fixed to the latter, A piece is cut out of the wooden support in order to 
permit of the free movement through it of the optical tube carrying the analysing 
nicol and the half-shadow calcite plate, the aperture being left sufficiently wide to 
enable the vernier and circle to be easily read through it with the aid of a pocket 
lens. Although the amount of rackwork provided wdth the analysing tube will not 
permit the objective of the polarising tube to be conveniently brought close up to 
the ground-glass diffusing screen of the monochromatic light apparatus, as the 
crystal would then appear very small when observed through the analysing tube, 
still the illumination suffers little from this cause, and is superior to that which is 
obtained by employing the diffusing screen in the form of a cap fitting on the end of 
the polarising tube. When the wdrole arrangement is moved up so that the metal 
foot is within a quarter of an inch of the end of the diffusing tube, and the two 
optical tubes are likewise approached as near as the rackwork will permit, an 
excellent illumination is obtained on generating the light in the lantern, far more 
brilliant than is usually obtained from a sodium flame, and ample to permit of the 
most accurate determinations of the directions of extinction for light of any wave¬ 
length from that of lithium up to that corresponding to G of the solar spectrum. 
Of course the arrangement is equally applicable when any other form of stauroscopical 
plate, such as that devised by Brezina, is employed instead of the half-shadow 
Calderon plate. 
For merely studying the phenomena exhibited by loose crystals or crystal plates 
in parallel polarised monochromatic light, the vertical arrangement first mentioned is 
naturally employed. 
The foregoing represent only a typical few of the applications of the apparatus 
for producing monochromatic light described in the earlier part of this communica¬ 
tion. Its use may be extended to all other cases in which it is desired to illuminate 
