364 PHYSICS. 



Laurent Las succeeded in ijroducing artificially magic mirrors of sil- 

 vered glass. Two kinds have been made; one made magic hy compress- 

 ing air behind it or by curving it in a frame, the characters being engraved 

 on it; and another, of any form whatever, heated in a particular way^ 

 by means of a metallic stamp having the characters upon it. If the 

 layer of silver is thin, the characters are bright if the silver is on the 

 side opposite to the screen, but are dark if the silvered side is toward 

 the screen. {J. Phys., November, 18S1, x, 474.) 



Klein has observed a complete change in the optical image of boracite 

 by heating it. The boundary lines of the optical fields prove variable 

 with temperature, and often wholly disappear, perhaps reappearing in 

 quite different places. He concludes that this mineral does not owe its 

 origin to a twin-like formation of parts of lower symmetry, but is regu- 

 lar, and produces simple individuals; and the optical properties, appar- 

 ently in sharp contradiction to this, are really duo to tensions produced 

 in growth. These divide the crystal into parts of different tension of 

 which the stronger sometimes suppress the weaker for certain tempera- 

 tures and positions of the crystal. Analcime shows similar properties. 

 {Nature, June, 1881, xxiv, 112.) 



Cassani has devised a neat optical illusion produced with mirrors. 

 An observer stands opposite a concave mirror supported at a slight 

 slant, at a distance greater than the radius of curvature, and receiving 

 no other light than that reflected from his face, which is illuminated by 

 a dark lantern. A small plane mirror is placed in a position nearer the 

 concave mirror than the observer and sloping in the opposite direction, 

 concealed from his view. On looking obliquely upward the observer 

 seems to see a plane mirror larger than the other, with his direct image 

 in it. The illusion is more complete if the mirror has an ornamented 

 frame. {Nature, February, 1881, xxiii, 372.) 



Montigny has proposed a method for measuring the index of refi'ac- 

 tion of liquids, founded ui^on the apparent displacement which the image 

 of a body immersed in a transparent liquid undergoes when the light- 

 rays reaching the eye issue oblique to the horizontal surface of the 



liquid. {Bull. Acad. Blelege, II, xviii, ; J. Phys., January, 1881, 



X, 50.) 



Hurion has suggested an apparatus for simplifying the method of 

 determining indices of refraction by means of Talbot's fringes, proposed 

 by Mascart. Its object is to vary the level of liquid in one of two com- 

 partments, so as to displace the fringes by a known value. The displace- 

 ment and change of thickness being known, the index is easily calcu- 

 lated. {J. Phys., April, 1881, x, 154.) 



Damien has measured the index of refraction of water when in a state 

 of surfusion, by the ordinary method, with the prism. The three hydro- 

 gen lines were measured. From the figures obtained he concludes that 



