NATURAL PHILOSOPHY. 145 



level with the outer surface. By these means, the structure of the 

 scale of the podura, and the different characters of its inner and outer 

 surfaces, are rendered distinctly visible. London Athenccum, June. 



NEW SOLID EYE-PIECE. 



REV. J. B. REA.DE stated to the British Association, at Edinburgh, 

 that, by simply filling the eye-piece with water, he had been able to 

 get rid of the two well-known defects of the common negative eye- 

 piece, a play of false light and the formation of a false image, or, as it 

 is generally termed, a ghost, of a planet or star. The addiition of the 

 water causes the ray of light to pass to the eye without suffering any 

 inner reflection from the surfaces of the lenses of the eye-piece. It also 

 makes the eye-piece positive instead of negative, while, at the same 

 time, the magnifying power remains nearly the same, the magnitude 

 and flatness of the field are preserved, and the achromatism is not dis- 

 turbed. It is, however, desirable to make the inner surface of the 

 field-lens a little convex, as the ray now passes out of glass into water, 

 and not into air. This eye-piece has been tried, and is very highly 

 spoken of. To avoid some little trouble arising from the use of water, 

 the author proposes to substitute glass or rock-crystal for the water, 

 and to cement the surfaces together with Canada balsam ; in this case, 

 the inner surfaces of the eye and field-lens must have a diminished 

 radius of curvature. London Athenaeum, Aug. 



CHROMATIC STEREOSCOPE. 



SIR DAVID BREWSTER has described to the Royal Scottish Society 

 of Arts a chromatic stereoscope, which consists of one lens 2k inches 

 in diameter or upwards, through the margin of which each eye looks 

 at an object having two colors of different refrangibility. The effect 

 of this is to cause the two parts of the object thus differently colored 

 to appear at different distances from the eye, just as in the lenticular 

 stereoscope the two parts of an object that are nearest to one another 

 in the double picture rise in relief, and give the vision of distance as 

 of a solid figure. The instrument may consist of two semi-lenses, con- 

 vex or concave, or of two prisms with their refracting angles placed 

 either towards or from one another ; and the effect is greatly increased 

 if the lenses or prisms have high dispersive powers, such as flint-glass 

 or oil of cassia. Civil Engineer and Architect's Journal, Feb. 



NEW OPTICAL INSTRUMENTS. 



JAMESON'S Philosophical Journal, for January, contains a description 

 of the following new optical instruments. 1. Polarizing spectacles, 

 to enable naturalists and others to distinguish objects beneath the sur- 

 face of the water. This instrument consists of a pair of Nichols 

 prisms, so adapted as to prevent the transmission to the eye of the 

 horizontally polarized ray reflected from the water, by which means 

 the glare that prevents the light from penetrating below the surface is 



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