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XV. An Instrument oj Pi'ecision for Producing Monochromatic Light of any desired 
Wave-length, and its use in the Investigation of the Opticcd Properties of Crystals. 
By A. E. Tutton, Assoc. R.C.S., Demonstrator of Chemistry at the Roycd College of 
Science, South Kensington. 
Communicated hy Prof Thorpe, F.R.S. 
Received January 11,—Read February 1, 1894. 
In the optical investigation of crystals it is of great advantage to command a ready 
means of illuminating the field of the observing instrument with light of any desired 
wave-length. The red, yellow, and green monochromatic light emitted by incan¬ 
descent salts of lithium, sodium, and thallium has hitherto been considered sufficient 
for most crystallographical investigations. The disadvantages of employing such a 
source of monochromatic illumination are threefold. In the first place it is difficult 
to remove the last traces of the relatively more powerfully illuminating sodium from 
the lithium salt employed. The admixture of yellow with the red light is a very 
great inconvenience when determining refractive indices hy the method of total 
reflection and when measuring the optic axial angle of biaxial crystals by observations 
of the separation of the hyperbolic brushes of the interference figures. In the latter 
case, owing to more or less dispersion of the axes for light of different wave-lengths, 
the effect of the admixture of even a little of the highly illuminating yellow sodium 
light with the red lithium light is to diminish the definition of the brushes, the inter¬ 
ference figures for the two colours being superposed, and thus to destroy the possibility 
of accurate measurement of the separation of the axes for lithium light. In the 
second place, the poisonous nature of the fumes of the volatile thallium salts renders 
it imperative that the green flame should be produced in a draught cupboard, and all 
observations conducted in front of it, a condition which it is frequently inconvenient 
to fulfil. The third and most weighty objection to this mode of producing mono¬ 
chromatic light is that it confines the observations to three wave-lengths, at con¬ 
siderable intervals apart, ceasing, however, with the yellowish-green, and leaving the 
blue end of the spectrum out of consideration altogether. For substances wdiose 
crystals exhibit very slight disjDersion of the optic axes this may, perhaps, be conceded 
to be sufficient, although, even in these cases, the observations cannot be considered 
as complete. For the numerous substances, however, whose crystals are endowed 
with sufficient dispersion to exhibit considerable differences of optic axial angle, and 
MDCCCXCIV. —A. 6 A 19.12.94 
