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XXXVILI. 
ON AN IMPROVED METHOD OF IDENTIFYING CRYSTALS 
IN ROCK SECTIONS BY THE USE OF BIREFRINGENCE. 
By J. JOLY, M.A., ScD., F.R.S., Hon. Sec. R.D.S., Professor 
of Geology and Mineralogy in the Univsrsity of Dublin. 
[Read Apri 17; Received for Publication Aprit 19; Published Juny 6, 1901.] 
OpricaLLy uniaxial minerals when compared in sections of equal 
thickness give the highest interference colours in sections parallel 
to the optic axis and biaxial substances in sections taken at right 
angles to the optical normal ; in other words, in sections containing 
the greatest and least axes of elasticity, and, therefore, containing 
the optic axes. 
From these principles is derived one of the most valuable 
means of diagnosis at the disposal of the petrologist ; for not only 
are many substances sharply differentiated from others by the 
brightness and colour of their interference tints, but this character 
is generally ascertained after a brief examination. 
A section of the substance under investigation is sought which 
shows the highest interference tint according to Newton’s scale of 
colours. This tint will appear in greatest intensity when the 
principal section of the plate is at 45° with the planes of vibration 
of polarizer and analyser. If the crystals of the substance are at 
all numerous in the rock slice, the sections exhibiting the highest 
tint will almost certainly approximate to the maximum interference 
tint proper to the specific mineral and the thickness conferred 
upon it in the rock slice. We may now, by the use of M. Michel 
Lévy’s Comparateur, estimate the retardation giving rise to the 
observed tint, and, dividing by the thickness of the rock slice 
{expressed in the same units of length as are used to measure the 
retardation), determine the birefringence; or we may, in many 
cases, proceed still more directly to find this important constant 
by reference to the valuable chart which MM. Lévy and Lacroix 
have provided in their work : “Les Minereux des Roches.” It will 
SCIENT. PROC. R.D.S., VOL. IX., PART Iy. 2N 
