1895.] Mineralogy. 991 
with adjusting the crystal or grinding. The grinding table is supplied 
with nine different laps suited to minerals of different degrees of hard- 
ness and to artificial crystals. The apparatus may be driven by a 
small motor, the current from three pint bichromate cells being ample. 
These instruments are constructed by Messrs. Troughton and Simms, 
the smaller instrument at a cost of £40, and the larger one, which is 
adapted for use of mineralogists and chemical crystallographers alike, 
at a cost of £60. 
An Instrument for Producing Monochromatic Light of 
any Wave Length.—The same author has constructed an instru- 
ment to furnish strong light of any desired wave length, which wave 
length may be changed at willt The source of light is an oxy-coal 
gas lime lantern and the dispersive apparatus a specially constructed 
spectroscope in which the telescope is replaced by a collimator tube and 
slit exactly like the one on the side of the instrument toward the 
source of light. The prism has a refracting angle of 60°, is pre- 
pared from heavy flint glass, and is rotated on a graduated circle so as 
to allow any desired wave length of the spectrum to pass through the 
exit slit. This is diffused by a plate of ground glass before it enters 
the goniometer, total refractometer, or axial angle apparatus, in which 
it is utilized in determining the index of refraction or the size of the 
optical angle. It is thus possible to extend indefinitely the measure- 
ments to show the amount and character of the dispersion of crystals, 
while greatly facilitating the measurements themselves. By replacing 
the exit slit by diaphragms having two or more slits at proper distances 
apart, composite light made up of any desired wave lengths may be 
employed, which is very useful in studying crystals with crossed axial 
planes like brookite. 
Other Mineralogical Apparatus.—Wolff® gives detailed in- 
structions for making diamond saws suitable for section cutting, also 
directions for sawing sections so thin that only a small amount of sub- 
sequent grinding is necessary.—Federow® describes the simplest form 
of his universal microscope stage, which is specially adapted for rapid 
petrographical determinations. Atthesame time he advocates length- 
ening the heretofore circular opening in his ebonite section holder. 
* Philosophical Transactions, Vol. 185, (1894), A, pp. 913-941. 
5 Am, Journ. Sci., XLVII, pp. 355-358, (1894). 
* Zeitsch. f. Kryst., XXIV, p. 602. 
