V. REFLECTION, ETC. 195 



908. Apparatus designed to exhibit Double Reflection, 



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

 crystal reaches 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 ciystal rests is turned round its vertical axis, four, three, or two 

 images of the orifice will be seen formed by the two 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. 



9O8a. A large Mass of Iceland Spar, polished to exhibit 

 the optical properties of the crystal in directions parallel and per- 

 pendicular to the optic axis, and in directions perpendicular to the 

 cleavage plane 100, and to the plane 122 correlative with it. 

 Worked and polished by Mr. Ahrens. 



Professor N. I. Maskelyne, F.R.S. 



909. Extra Dense Flint Prism, of 60, surface 4 by 3 in. 



A. Hilger. 



910. Iceland-spar Prism, of 60, showing single refraction 

 for any line in the spectrum. A. Hilger. 



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

 Two triangular prisms are joined together at their hypothenuse; 

 while the direct observer looks through the cube formed by the 

 union of the two prisms, an incidental observer sees in the 

 hypothenuse the same reflection as though in a mirror. The 2 J 

 lens serves to show the reflection smaller and reversed. 



M. Cretes, Paris. 



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

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

 refraction becomes annulled : (1515=0). When placed basis 

 upon basis, 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 in reverse ways and in equal volume. M. Cretes, Paris. 



912a. Three Rectangular Prisms, crown glass. 



M. Lutz, Paris. 



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



M. Lutz, Paris. 



912c. Prisms for Camera. M. Lutz, Paris. 



912d. Prisms with Compartments. M. Lutz, Paris. 



N 2 



