( 734 ) 



impossible entirely to exclude difFiise light^ — probably due to the diffu- 

 sion on the faces of and inside the prisms, so that we shall always 

 have to take account of its presence, even though it be only to a 

 very slight degree. In fact, we have not investigated any apparatus 

 or prism, in which the disturbing influence of this phenomenon was 

 not more or less felt. The question whether a certain diffusion still 

 occurs with a really macro-homogeneous, optically "empty" refracting 

 medium, is difficult to solve, and must for the present be considered 

 a pending problem.^) 



The "vignettation" amounts on an average to 25 "/„, as may be 

 observed by accommodating on the square objective diaphragm. When 

 the apparatus was used as a monochromator the intensity of the light 

 came up to what we expected ; with sunlight it is still from 5 to 

 10 times higher (according to meteorologie circumstances) than 

 with an arc-lamp crater projected on the entrance slit. Accordingly 

 Avitli monochromatic light of great purity even polarisation apparatus 

 of very slight transmitivity may be used. When thus applying the 

 instrument to illuminative purposes the entire path of the beam from 

 the source of light on to the retina, and especially its divergence, 

 ought to be carefully adapted to that j)art which lies within the 

 apparatus, if all the possible benefit is really to be derived from it. 



Physics. — ''The injluence of temperature and magnetisation on 

 selective absorption spectra', II. By Prof. H. E. J. G. du Bois 

 and G. J. Elias. (Communication from the Bosscha Laboratory). 



^ 12. Since our former communication (These Proc. Febr. p. 578) 

 the cryomagnetic arrangement was further improved in some respects 

 in order to obtain a stronger field, and to diminish the inconvenient 

 formation of rime. The truncated end-planes of the conic polar 

 pieces had a diameter of 6 mm., the split cores ') a diameter of 

 3,5 mm.; the width of the slit at the end was from 0.4 to 0.6 mm., 

 the slit being wedge-shaped so as to fit the convergence and diver- 

 gence of the beam of rays between two lenses ; it was arranged 

 in such a way that the whole surface of the grating was illuminated, 

 so that the theoretical dissolving power, — amounting to about 

 100.000 — had its full effect. Subsequent in the direction of the 

 rays was a doubled quarter-wave plate with horizontal demarca- 



1) G. A. LoBRY DE Bruyn and L K. Wolff, Rec. d. Trav. Ghim. 23, p. 155, 1904; 

 L. Mandelstam, Physik. Zeitschr. 8, p. 608, 1907; M.Planck, ibid. 8, p. 906, 1907. 



2) H. DU Bois, Zeitschr. fur liislr. Kunde 19 p. 360, 1899. 



