370 PHYSICS. 



meter wide, if they are placed exactly in the same plane; but the ad- 

 justment is difficult. If, however, there is placed behind the narrow- 

 black mirror a second silvered mirror perpendicular to the first one, 

 then the phenomena appear, the image of the first in the silver mirror 

 answering for a second black mirror. By covering the surface of a 

 black mirror with lines of India ink, the spaces being equal in width to 

 the lines, beautiful grating spectra are obtained by a suitable incidence. 

 {Carl Rep., xvi, 454; J. Phys., March, 1881, x, 129.) 



Fuchs has described anew interference photometer, in which no polar- 

 ization of the rays at right angles is required. It consists simply of two 

 similar isoceles glass prisms joined by their basal surfaces, which inclose 

 an air-layer variable in thickness by pressure. A diaphragm reaches 

 out in i^rolongation of the surface of junction. The observer looks 

 obliquely toward this surface, and sees one illuminated surfiice directly 

 through the double prism, the other by reflection at the air layer. One 

 source of light is fixed and the other is displaced till the interference 

 bands disappear. ( Wicd. Ann., II, xi, 4G5 ; J. Fhys., March, 1881, x, 127 ; 

 Nature, January, 1881, xxiii, 278.) 



C. S. Peirce has communicated a note on the width of the rulings on the 

 closest-ruled diflfraction-plates made on Mr. Eutherfurd's engine. He 

 finds that these plates have a mean width of ruling varying in different 

 specimens from G8078 to 68082 lines to the decimeter, at 70° F. A line 

 in the solar spectrum has been selected for the measurement of wave- 

 length whose minimum deviation with one of the above plates in the 

 spectrum of the second order is 45° 01' 50". The author suggests this 

 line as a standard of reference, since it is possible to deduce from the 

 minimum deviation of this line j)roduced by a given plate the mean width 

 of the rulings on it; and consequently the wave-length of any other 

 line whose deviation has been measured with it. Peirce finds the wave- 



o 



length of this line to be 5024825 ; Angstrom gives it 562336. {Nature, 

 July, 1881, xxiv, 262.) 



Cornu has constructed a polarizing prism made of a single film of Ice- 

 land spar, fixed with Canada balsam between two flint-glass x)risms. 

 The polarization is far from perfect, however, and the field is very nar- 

 row, so that the instrument, though of interest from a theoretical point 

 of view, is of little or no practical value. {Nature, September, 1881, 

 xxiv, 504.) 



Glan has devised a new i)olarizing prism, in which the total reflection 

 takes place on air, as in the Foucault jirism ; but the face of the i^rism 

 is perpendicular to the incident beam and the axis of the spar is parallel 

 to the diagonal section between the two halves of the prism. To trans- 

 mit a luminous beam of section unity the length required in the new 

 prism is 1.141, that of the Foucault being 1.228, and that of the JSicol 

 3.281. The maximum angle of the polarized bundle is 7° 56' ; hence the 

 rays must be made parallel by a collimator. {Carl. Bep., xvi, 570; J. 

 Fhys., April, 1881, x, 175.) 



