PLATES AND PRISMS OF CRYSTALS OF ARTIFICIAL PREPARATIONS. 889 
Tt will be evident that this mode of procedure can, at the best, only furnish 
plates and prisms whose surfaces are approximately plane and true to the desired 
directions. For the difficulty must at once be apparent of holding a small crystal, 
perhaps only one or twm millimetres in its longest dimension, so that a certain plane, 
judged by reference to the developed faces of the crystal, is parallel with the grinding 
plate. Moreover, even after long practice, it is impossible, other than exceptionally, 
to grind a truly plane surface. The use of a very slightly convex grinding-plate 
helps but little to counteract the effect of an involuntary turn of the wrist. Tt is a 
most disagreeable, but frequently-recurring experience, to grind and polish, after con¬ 
siderable trouble, a smooth and apparently plane surface without accident from 
fracture, and then to find upon goniometrical examination that it is perhaps five or 
more degrees out of the desired direction. It also often happens that many hours 
are wasted by the fracture of crystal after crystal during the grinding. It will thus 
be seen that the preparation of a large number of sections and jjrisms by the current 
method, for the purposes of an extensive investigation, is attended by a prodigious 
amount of labour, and is a severe tax'upon the patience of the investigator, wffiile the 
results can rarely be more than approximate. 
The instrument now to be described is the result of an attempt to replace these 
wearying and approximate methods by a method of precision, which shall eliminate the 
fatigue of hand work, while assuring that the ground surface shall be truly plane and 
shall lie in the right direction. The attempt has met with success, and it is possible 
by the use of the instrument to grind and polish a truly plane surface in any direction 
in a crystal, so as to be true to that direction to within ten minutes of arc, an amount 
of possible error which wmuld exercise no measurable influence upon the values of the 
optical constants. Moreover, this result may be achieved in a small fraction of the 
time hitherto required, and with only the very slightest risk of fracturing the crystal. 
An arrangement is also provided by which a second surface may be ground parallel, 
with a like degree of accuracy, to the first. It is also found easily possible, by the use 
of it, in cases where the double refraction is low, so that fairly thick sections are 
required to exhibit small rings in the interference figure, to grind and polish two pairs 
of parallel faces, perpendicular to the first and second median lines respectively, upon 
the same crystal. It is likewise ati easy matter, and can be made the usual course 
of procedure in the case of biaxial crystals, whatever the amount of double refraction, 
provided the crystals are not too minute, to grind a pair of prisms in such directions 
upon the same crystal as will afford all three refractive indices. Indeed, when 
crystals of low double refraction and of three or more millimetres diameter are avail¬ 
able, it is not difficult to grind two section-plates and two prisms upon the same 
crystal, from which the whole of the optical constants may be obtained. The sections 
and prisms furnished by the instrument possess the further advantage of being so 
highly polished as to enable them to be employed directly, without the intervention 
of cover glasses, for the purposes of the determinations of the optic axial angle and 
MDCCCXCrV.—A. 5 X 
