692 SUMMARY OF CURRENT RESEARCHES RELATING TO 



shall be grateful if informed of the results of any experiments tried by 

 them." 



Gundlach's Substage Refractor.* — This apparatus, intended for 

 measuring the aperture angle of wide-angled objectives, consists of a 

 small crown- glass cube, with sides about T \ inch. One face is opaque, 

 the one opposite and the two others opposite each other, polished. 

 The cube is made to adhere, by means of a suitable homogeneous 

 medium, to the front surface of the objective by the polished surface 

 opposite the opaque side. Then a ray of light must enter each of the 

 polished side surfaces in the plane described by the optical axis of 

 the objective and a line perpendicular to those polished surfaces, and 

 at such angular inclination to the optical axis that it will pass 

 through the objective close at the edge of its aperture, and emerge 

 from it in the direction of the optical axis. 



The angle described by the refracted rays inside the crown-glass 

 cube, is equal to the crown-glass aperture angle of the objective, 

 and is : — 



a being half the angle described by the two rays before entering the 

 cube, r the refractive index of the crown glass, and n the crown-glass 

 angle of the objective. 



Silvered Convex Lenses v. Concave Mirrors.t— Mr. C. V. Boys 

 points out that convex lenses silvered at the back make excellent and 

 easily-constructed concave mirrors. Since both surfaces conduce to 

 bring the light to a focus flatter curves may be used than are neces- 

 sary for a plain concave reflector of the same focal length ; also since 

 the two surfaces are not parallel false images are not produced, so that 

 the advantage of glass silvered at the back remains without the usual 

 disadvantage. 



Binocular Vision in the Microscope.^— Professor C. Cramer, in 

 connection with a description of Prazmowski's binocular eye-piece, 

 discusses the conditions of stereoscopic binocular vision in the Micro- 

 scope. In particular he points out the error of the views of Nageli 

 and Schwendener that the depth of the field of view is of only secon- 

 dary importance to the stereoscopic effect, a view which they attempt 

 to support by the fact that in the ordinary stereoscope the two pictures 

 are perfectly plane, but yet produce the impression of solidity. These 

 pictures require, however, to be taken from different points of view, 

 or no stereoscopic effect whatever will be produced. Microphoto- 

 graphs of statuary, &c, do not appear to be more solid when 

 observed with a stereoscopic Binocular than with a single eye. The 

 author further describes the appearances, by the left- and right-hand 

 halves of an objective respectively, of oil-globules and air-bubbles in 

 water by transmitted light and a small cylindrical opaque object, as 

 establishing to what in fact the stereoscopic effect is due. He also 



* Amer. Mon. Micr. Journ., iii. (1882) pp. 142-3 (1 fig.). 



t Phil. Mag., 1882. 



% Vicrteljahrsschr. Naturf. Gresell. Zurich, xxiv. (1879) pp. 95-106. 



