PHYSICS. 267 



the mirrors, tbis is true of projected images only when the angle between 

 the mirrors is an even snbmultiple of the circumference. For this projec- 

 tion the light should be concentrated upon the object, say a small hole 

 in a blackened screen. Behind this screen are the blackened mirrors, 

 one horizontal, and behind these a lens for producing the image on the 

 screen. If the inclination of the second mirror be varied, the number 

 of the images increases by doubling one of them. — {J.Phys., ix, 11. Jan- 

 uary, 1880.) 



Blcekrode has measured the indices of refraction of cyanogen, carbon 

 dioxide, and ammonia, liquefied in a Faraday tube having parallel sides 

 at one end. He used the method of determining the index by the dis- 

 placement of an object seen with a microscopic objective when the space 

 at first empty is tilled with the liquefied gas. The index of cyanogen, 

 at 13°, for the sodium line, was found to be 1.320; of carbon dioxide at 

 19°, 1.163; of ammonia at IT^.S, in sunlight, 1.314. The index of zinc- 

 ethyl was obtained as 1.489. The above value for CO2 is very near that 

 obtained by Brewster in 1826, for a liquid contained in a cavity in Bra- 

 zilian topaz, undoubtedly liquid CO2. — {Ann. Pliys. Chcm., II, viii, 400, 

 1879 ; J. Phys., ix, 141, April, 1880.) 



Bertin has described an inverting prism devised by Duboscci for use 

 with the lantern, in cases where the object cannot itself be inverted. It 

 consists of a right-angled isosceles i^rism whose hypothenuse is placed 

 parallel to the axis of a convergent beam of light, its center coinciding 

 with the focal point. Under these circumstances the beam is totally 

 reflected irom the base of the prism and inverted. In practice, two of 

 these prisms are placed in the beam from the lantern, with their bases 

 together. The use of a prism for this purpose is not new ; it is figured 

 in Brewster's Optics (London, 1853, p. 454,) where it is described for the 

 same purpose. More recently (J. Fr. Inst, Ixiii, 375, 1872) Zeutmayer 

 has constructed an inverting prism for the microscope, which he has 

 also used' for the lantern, and which would seem to be more effective so 

 far as amount of light is concerned than the Duboscq arrangement. — 

 {J. Fhys., viii, 336, October, 1879.) 



Gariel has proposed some ingenious mechanical models for illustrat- 

 ing the relations of the foci of lenses. Two rods are articulated on a 

 line perpendicular to the axis of the lens — drawn in section — at a point 

 near its circumference. These rods are attached near their ends to op- 

 posite parts of a cord which moves over pulleys attached to the frame. 

 Sincje the angle between the rods remains constant, by moving the cord 

 the variations in the positions of intersection of the rods with the axis, 

 i. e., the foci, may be clearly shown. A noteworthy modification of 

 this model enables the optical i)heuomena of the eye, accommodation, 

 emmetropia, myopia, and hypermetropia, to be shown distinctly to an 

 entire class. — (</. Fhys., ix, 340, October, 1880.) 



Stevenson has suggested an instrument for exploring dark cavities 

 which are inaccessible to direct light. It is simply a train of i)risms by 



