320 Intelligence and Miscellaneous Articles, 



and the spark-spectrum is merely one of temperature ; but so far 

 surely the arc-speetra photographs have been more suitable for the 

 comparison of spectra. Eegarding magnesium and zinc, I must 

 still say that the resemblance is much less marked than between 

 zinc and cadmium ; and it is well to note that the recent work of 

 Kayser and Eunge shows conclusively that magnesium is not 

 related so closely to zinc and cadmium as it is to barium and the 

 elements on the other side of the second column of Mendelejeff's 

 table. 



Very respectfully yours, 



Joseph Sweetman Ames. 

 Johns Hopkins University. 



MAGNETO-OPTIC PHENOMENON. 

 To the Editors of the Philosophical Magazine and Journal. 

 'Gentlemen, Delft, June 23, 1891. 



Having read in the Philosophical Magazine for April 1891 a 

 translation, by Mr. James L. Howard, of a paper of mine in Wiede- 

 mann's Annalen, I venture to make some remarks on this trans- 

 lation, the exactness of which is highly to be admired. I hope 

 that you will find occasion to correct these small errors in a fol- 

 lowing Number in order to prevent misapprehension. The first 

 and seventh apply to misprints which have also crept into the 

 German translation and which are corrected in "Wiedemann's 

 Annalen, vol. xliii. p. 680. 



I am, Gentlemen, 



Tour obedient servant, 



E. Sissingh, 

 Lecturer on Physics at the Polytechnic 

 Institute at Delft. 

 Page 294, line 2 from top, for does not pass read passes. 



— 296, end of footnote, for perpendicular to the plane of inci- 



dence read perpendicular to the polarizer. 



— 298, last line of text, for round the edge of a circular plat- 



form read longitudinally upon an iron frame. 



— 299, lines 17 and 18 from top, for these two lines read in- 



serted, just as in the case of the polarizer, on a pillar, 

 which can be moved on a cross-piece along the frame. 



— 300, line 14 from top, for inclined to each other read inclined 



to its length. 



— 302, line 5 from top, read whose mean value was approach- 



ing to zero. 



— 308, line 11 from foot, for square centimetres read square 



millimetres. 



— 308, line 5 from foot, for the differences were ^ wave-lengths 



read the differences were reduced to one half from one 

 angle to the next. 

 - — 311, line 5 from foot, for derived from read assumed in the 



