470 SCIENTIFIC EECORD FOK 18B9. 



face. It revolves in front of a small chamber lined witii black velvet, 

 and the proportion of black to white is determined by the size of the 

 sector cut from the white disk. {Nature, November, 1884, xxxi, 58.) 



Hofiert has devised a new apparatus for producing color combinations, 

 which consists of a flat box irregularly hexagonal in shape, on one side 

 of which six prisms are placed, arranged in two pairs and all set at the 

 angle of minimum deviation. The first prisms have their refracting 

 edges in contact, and, by means of a screen in which is a small rectangu- 

 lar aperture, small equal strips of the adjacent faces of these prisms are 

 visible from an eye-piece which is simply a tube of brass in which is 

 a slit one-tenth of an inch wide. If light entered through the eye- 

 piece, each set of the prisms would deviate it about 150°, one to the 

 right, the other to the left, aftd each beam would then fall on a lens of 

 about 10 inches focal length, the two spectra thus i>roduced being 

 brought to a focus on the sides of the box immediately to the right and 

 left of the eye-piece. At these points, on each side, are placed three in- 

 candescent wires ; so that, conversely, if these be the sources of light, the 

 rays follow an inverse course, and the corresponding half of the aperture 

 in the screen is seen illuminated with a color which will depend on the 

 position of the incandescent wire. {PJiil. Mac/., August, 1884, V, xviii, 

 81.) 



4. Interference and Polarization. 



Kissliug has investigated the influence which foreign admixtures 

 exert on the formation of fog in moist air, in the course of which he has 

 observed a series of diffraction phenomena, the law of whose formation 

 can only with difficulty be made to agree with Fraunhofer's law. He 

 finds that in general the law of Aitken is true, and that when aqueous 

 vapor is condensed in the air it always takes place on some nucleus. 

 In the ordinary unfiltered air of a dwelling-room, when the space in 

 which diffraction occurs is but slightly cooled, the fog is so strong that 

 it greatly enfeebles even a powerful source of light. If this air be 

 gradually admixed with filtered air, the formation of fog gradually di- 

 minishes, while at the same time phenomena of diffraction set in, the in- 

 tensity of whose color increases until the quantity of vapor has sunk to 

 a definite though extremely small amount. Small admixtures of sul- 

 phurous oxide and of ammonia with the unfiltered air of a room pro- 

 duce so strong a fog that any action of diffraction ceases. {Phil. 

 Mag., August, 1884, V, xviii, 160.) 



Madan has called attention to a simple method of producing the in- 

 terference pTienomena known as Ohm's fringes. Ohm himself directs 

 that two ,)lates of equal thickness are to be cut from a uniaxial crystal, 

 their parallel surfaces making an angle of 45° with the optic axis. If 

 now one of these be placed on the other in such a position that the optic 

 axes lie in the same plane but upon opposite sides of the normal common 

 to the two plates, and the combination be held in a convergent beam of 



