484 SCIENTIFIC RECORD FOR 1882. 



brief or the intensity of these rays is snflSciently reduced, a greenish 

 light where they fall on the screen, which faded much more ra])idly 

 than the deep bine, and after a short time became relatively dark, lu 

 photography, as in phosphorescence, there may be in certain cases an 

 antagonistic action between the more and less refrangible rays, so 

 that the withdrawal of the latter might promote the effect of the former. 

 In a camera corrected for chemical rays, w^hen directed to a dark object 

 on a bright ground, there is a sharp transition from light to dark for 

 rays in focus, but a gradual one for rays out of focus. Just at the out- 

 line of the object there would be half illumination for rays out of focus, 

 and the illumination would go on increasing until the full intensity is 

 reached at a distance equal to the radius of the circle of diflusion. If, 

 now, the rays of low refrangibility tend to oppose the action on the 

 sensitive plate of those of high refrangibility, acting negatively, just 

 outside the outline, the active rays, being sharply in focus, are in full 

 force while the negative rays have not yet acquired their full intensity. 

 At an equal distance from the outline on the dark side the positive 

 rays are absent, and the negative rays, having nothing to oppose, do 

 nothing. {Proc. Boy. Soc, May 25 ; Nature, June, xxvi, p, 142.) 



3. Dispersion and Color. 



Ayrton has presented a paper to the London Physical Society de- 

 scribing the latest form of his dispersion photometer. A concave lens 

 is used to disperse the stronger light, thus obviating the necessity of 

 liutting it at a distance. The illuminating j^owers are compared by 

 Rumford's method, the intensities of two shadows of a rod thrown on a 

 white screen of blotting paper being adjusted to equality. A sperm 

 candle is used as the standard, placed on a stand sliding on a graduated 

 rod at an angle with the frame of the instrument which carries the lens 

 and is also graduated. The stronger beam is reflected to the lens by a 

 small mirror inclined at 45° to a horizontal axis, about which it can 

 rotate. Collateral observations are taken through red and green glasses 

 to get a better estimate of the i>ower of the light. {Nature, March, 

 XXY, p. 426; Phil. Mag., July, V, xiv, p. 45.) 



In order to show the focus after refraction through a prism, Crova 

 replaces the slit of a spectroscope by a plate of silvered glass having two 

 lines at right angles drawn through the silvering, the quadrants being 

 filled with fine lines drawn in various directions. When illuminoted 

 with sodium light, and viewed in the observing telescope, a sharp image 

 is obtained by these lines, both horizontal and vertical, when at mini- 

 mum deviation, but if the prism be displaced, the image of the lines 

 perpendicular to the i)rincipal section becomes indistinct. These phe- 

 nomena may be projected on a screen by suitable modifications in the 

 method. {J.Pkys., February, IT, i, p. 84."i 



Chappuis has examined carefully the absorption spectrum of ozone, 

 which he says characterizes this gas better than any other of its phys- 



1 



