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378 OPTICAL PROJECTION they Lave to be focussed by another power, as they appear at the back of the last lens. All sorts of focal arrangements have been employed, but I prefer that described on p. 343, for the sake of the easy range of foci and scale which it gives. The whole apparatus can be adjusted, and a crystal focussed, in less than a minute, both axes of selenite being easily shown in one field. A great variety of crystals are prepared by Messrs. Dr. Steeg and Keuter, of Homburg vor der Hohe, who practi- cally supply the scientific world with these objects, and from whom a list can be obtained. Their crystals are mounted in plates of cork If inches square. For home-made plates or mica preparations, I myself prefer the usual microscopic 8x1 slips, because stored so easily in the usual racked boxes. But a stage with springs, as shown in fig. 190, will accommo- date all alike with equal facility. In demonstrations, the front optical apparatus is fixed so as to leave about an inch between the two convergent systems ; a crystal is inserted, and then brought up close to the second system by racking out the ordinary focal power. The fringes are then focussed by the rack and pinion on the front. 1 Plates of some bi-axial crystals crossed are very fine. Mica is easily prepared thus. There should be at least one specimen of crystals in which the rings for red light are at right angles to those for blue in the same crystal. Such are Brookite—which is however expensive (for a good one) and shows considerable red or orange colour—and the triple tartrate of soda, potash, and ammonia, which is clear. Unfor- tunately it, like many ' soft' crystals, gradually oxidises when mounted in balsam, and thus becomes cloudy. A mounting 1 If the apparatus is properly adjusted, the crystal figures will be perfectly and evenly illuminated all over the disc. Should either the centre or the margin appear dark, the apparatus is faulty, unless the adjustable field-lens H (fig. 190) has been accidentally placed wrongly whilst varying its position to alter the focal power. Ample margin for all necessary adjustments is pro- vided in my apparatus. With only medium-angled bi-axials, the front lens of the collecting system may often be unscrewed ond removed with advantage,