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, 



