MONOCHROMATIC LIGHT OP ANY DESIRED WAVE-LENGTH. 
939 
the small graduated circle of the total-reflectometer, for as many positions (having’ 
reference to the axes of optical elasticity) of the crystal as it is desired to record 
observations for. 
Use o f the Instrument in Stauroscopical Ohsei'vations. 
The determination of the directions of extinction for different colours, exhibited by 
plates of crystals belonging to the monoclinic and triclinic systems of symmetry in 
parallel polarised light between crossed nicols, in order to ascertain the directions of 
the principal optical planes, by means of the stauroscope or a microscope arranged as 
such, may very readily be carried out with the aid of the monochromatic light 
apparatus described in this communication. It is only necessary to place the stauro¬ 
scope or microscope in front of the exit slit, so that the mirror usually carried by 
either of the instruments mentioned receives the line of monochromatic light at such 
an angle as to reflect it up along the axis of the instrument. The nearer the mirror 
is to the slit, the better the illumination. The diffusion into an even field of light is 
effected by use of a ground-glass diffuser carried as a cap at the lower end of the 
stauroscope, or fixed on the end of the polarising prism of the microscope. 
As, however, the use of the mirror causes some loss of light, by absorption at the 
back surface, the author prefers to conduct stauroscopical measurements with the 
stauroscope or microscope arranged horizontally, so as to be able to utilise the ordinary 
diffusing-tube of the monochromatic light apparatus, and to thus make use of the 
whole of the light issuing from the exit slit. Of course this necessitates that the 
crystal plate shall be firmly held on the stage, which is now vertical. Monochromatic 
light, however, is only required when making the actual measurements of the angles 
of extinction, during which the crystal plate is cemented upon the small rectangular 
glass plate provided for the purpose, one of whose edges coincides with a known 
reading of the circle and makes a goniometrically-ascertained angle with an edge of 
the crystal plate. Hence, for stauroscopical measurements there is no necessity for a 
vertical arrangement of the observing instrument, and the horizontal arrangement has 
the further advantage that the observer is then enabled to make the observations 
while seated, with the eye looking forwards at the normal height, a condition in which 
he is far better able to appreciate minute differences of light and shade in the two 
halves of the Calderon double-calcite plate, which is now generally employed in order 
to increase the accuracy of determinations of extinction, than when a vertical arrange¬ 
ment is employed. 
As the best crystallographical microscopes are provided with a swing arrangement 
which enables them to be placed' horizontally, there is no difficulty in so employing them 
for stauroscopical observations. It is only necessary to rotate the mirror out of the 
way, and to bring the instrument close up to the monochromatic light apparatus so 
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