PHYSICS. 017 



of Bimsen's scale (between C and D ^ E), its maximum being- near 44. 

 The exciting rays are almost exclusively visible rays, tbe most efficient 

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

 mum of absorption at this point. ( Wied. Ann., xxi, 422 ; J. Phys., ISTo- 

 vember, 1885, IT, iv, 535.) 



Abuey has described some lecture 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 prism 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 aj)- 

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

 different lights in any required proportion may be obtained. {Nature^ 

 July, 1885, XXXII, 2G3.) 



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 lami) 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 i>late. 

 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 i)henomena 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 per|)endicular to the center of a line joining these two points 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 o^jposite 

 phases and interfere. {Am. J. Sci., January, 1885, III, xxix, 5.) 



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

 tended investigation of the brilliantly colored iridescent crystals of po- 

 tassium chlorate sometimes obtained in the process of manufiicture. 

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

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

 color 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 



