PHYSICS. 017 



of Biiiisen's «cale (between C aiKl D J E), its iiiaxiimiiii being- near 44. 

 The exeitiug rays are abiiost exehisively visible rays, tlie most efficient 

 being those between E and h. Henee Iceland spar presents a maxi- 

 mum of absorption at this point. ( Wied. Ami., xxi, 422 -, J. Phys., No- 

 vember, 1885, ir, IV, 535.) 



Abney has described some lectured experiments on color mixtures, 

 employing for this purpose a modification of Maxwell's color box. The 

 spectrum, instead of being formed on a screen, is received upon a convex 

 lens, which forms an image of the face of the jnism on a screen. If all 

 the light from the prism falls upon the lens this image is colorless. But 

 by interposing a screen with a slit in the spectrum close to the lens, so 

 as only to allow light of a given color to fall on the lens, the image ap- 

 pears colored with that light. By using two or more slits mixtures of 

 different lights in any required projiortion may be obtained. {Nature, 

 July, 1885, XXXII, 263.) 



4. Interference and Polarization. 



Lummer has called attention to the fact that if a concave mirror, sil- 

 vered over its whole surface excepting a small place the size of the 

 pupil, be made to reflect the light of a lamp burning alcohol saturated 

 with salt, normally upon a plate of plane glass with parallel faces, the 

 eye placed at the opening when adjusted for distant objects will see 

 the plate covered with a great number of concentric rings whose centers 

 are at the foot of the normal from the center of the pupil on the plate. 

 If a telescope adjusted for parallel rays be substituted for the naked 

 eye, the incidence corresponding to each of these rings may be meas- 

 ured. This phenomena has been observed by Haidinger and studied 

 by Mascart, who has given a formula giving the difference of path of 

 the two interfering rays as a function of the incident angle. The author 

 suggests this method to detect want of parallelism in glass, the rings 

 being then irregular. ( Wied. Ann., xxiii, 49 ; J. Phys., February, 1885, 

 II, IV, 90.) 



Moreland has described a mechanical model which he uses to illustrate 

 the phenomena of interference. Two blocks cut in sine curves are at- 

 tached to two points by strings fastened to their ends. At a point ex- 

 actly perpendicular to the center of a line joining these two jioints the 

 waves are in the same phase and reinforce each other. By moving both 

 blocks either way a position is reached where they are in opposite 

 jduises and interfere. {Am. J. 8ci., January, 1885, III, xxix, 5.) 



Stokes has communicated to the Koyal Society the results of an ex- 

 tended investigation of the brilliantly colored iridescent crystals of po- 

 tassium chlorate sometimes obtained in the process of manufacture. 

 He regards it as conclusively proved that the seat of the color is in a 

 very lliin twin stratum, and he entertains little or no doubt that the 

 <*olor depends in some way on the different orientation of the planes of 

 polarization in the two components of a twin and on the difference of 



