V. REFLECTION, ETC. 227 



908. Apparatus designed to exhibit Double Reflection, 



which arises when a ray of light traversing a uui-axial or bi-axial 

 crystal readies the surface of contact of the crystal with the 

 surrounding medium. Arthur Hill Curtis. 



The incident light passes through a small orifice in the cap terminating 

 one of the tubes. If the eye be applied to the other tube, as the stage on 

 which the crystal resls is turned round its vertical axis, four, three, or two 

 images of the orifice will be seen formed by the tAvo rays which, refracted at 

 the upper surface, are (in general) each doubly reflected at the lower sur- 

 face. A Nicol's prism is added, which, though not essential to viewing the 

 phenomena, may be introduced into either tube to polarise the incident 

 light, or to examine the planes of polarisation of the reflected rays. 



I. Sphere of Calcite, 3J inches in diameter. 

 II. Polyhedron of Calcite, cut from a large rhombohedron 

 of that mineral, so as to represent the optical characters of the 

 crystal in directions perpendicular 



1 . To the pinakoid, and along the optic axis. 



2. To a prisrn plane, and perpendicular to the optic axis. 



3. To the cleavage planes (of the rhombohedron) {100}. 



4. To the plane {122} correlative to the cleavage rhombohedron. 

 (These were made by Mr. Ahreris.) ^ r f> N. S. Mashelyne. 



909. Dichroic Apparatus. A. ffilaer. 



910. Iceland Spar Prism, of 60, showing single refraction 

 for any line in the spectrum. A. ffilaer. 



91Oa. Two Hicol's Prisms. South Kensington Museum. 



911. Prism, with Double Reflector, of Dr. de Wecker. 

 Two triangular prisms are joined together at their hypothenuse; 

 while the observer looks directly through the cube formed by the 

 union of the two prisms, an observer looking obliquely sees in the 

 hypothenuse the reflection of the former as though in a mirror. 

 The lens serves to show the reflection smaller and reversed. 



M. Cretes, Paris. 



912. Prism, Movable, by Cretes. Two prisms of 15 

 each are placed in a setting. When placed base on edge, their 

 refraction becomes annulled: (15 15=0). When placed base 

 to base, their effect becomes added: (15 + 15=30). Between 

 these two extremes, an ascending scale of to 30 can be 

 obtained. The prismatic axis remains fixed, because the glasses 

 move equally in reverse ways. M. Cretes^ Paris. 



912a. Three Rectangular Prisms, crown glass. 



M. Lutz, Paris. 



912b. 32 Rectangular Prisms, flint glass of various sizes. 



M. Lutz, Paris. 



P 2 



