[ 139 ] 

 XVII. Intelligence and Miscellaneous Articles. 



ON THE FLUORESCENCE OF A SOLUTION OF FRAXINE WHICH AP- 

 PEARS UNDER CERTAIN CIRCUMSTANCES OF A YELLOW COLOUR. 

 BY THE PRINCE SALM-HORSTMAR. 



PROFESSOR STOKES has had the goodness to call my atten- 

 tion to the probability of the observation of a ^e//o2w fluorescence 

 of a solution of fraxine, in a test-tube placed in a case of blue cobalt 

 glass (see PoggendorfF's Annalen, vol. xcvii. p. 638), being due to 

 an optical illusion caused by the colour contrast. This suggestion 

 induced me to examine the matter more closely. 



If the experiment be so arranged that the test-tube containing the 

 solution be placed in a narrow opake case, closed at the bottom, and 

 having a single opening through which the daylight passes horizon- 

 tally, the blue fluorescence appears precisely as in direct daylight. 

 In a case of dark violet manganese glass, the solution also presents 

 the same blue fluorescence as appears in direct daylight. 



Hence it certainly follows that the yellow fluorescence mentioned 

 above was an optical illusion, caused by the fact that the eye only 

 recognized one part of the fluorescent light of the fraxine, that is, 

 only the yellow rays, because the much stronger blue light of the 

 glass case prevents the eye from recognizing distinctly the feeble 

 blue rays of the total fluorescence. The yellow rays of the fluorescent 

 light, although inferior in respect to quantity to the blue rays of the 

 entire fluorescence, are yet distinctly perceived, because the eye is 

 especially sensitive to them in the neighbourhood of the blue. 



The yellow fluorescent appearance is accordingly a part of the 

 total fluorescent light. Hence the appearance of the light in the 

 cobalt-glass case remains yellow upon an almost black ground if a 

 deep orange-coloured glass be held before the eye ; and it appears to 

 differ very little from the fluorescence of the uranium glass if the 

 orange glass be changed for a pure yellow one. 



The cobalt-glass case appears to be capable of application, not 

 only for finding quickly, and without the use of a prism, the yellow 

 constituent in a blue fluorescence, but also for judging to some ex- 

 tent of the relative quantities of the yellow constituent of the light in 

 totally different solutions having blue fluorescence. This is effected 

 by placing the several solutions side by side, cmteris paribus, in the 

 case. It is especially advantageous to cover one of the glass sides 

 of the case, that one namely which is turned towards the window, 

 with a piece of black paper, the height of which is almost equal to 

 the length of the test-tubes. The brilliancy of the yellow light is 

 thereby increased. — Poggendorff's Annalen, for April, 1858. 



NOTE ON INTERNAL CONICAL REFRACTION. BY DR. P. LECH. 



A simple means of observing and measuring the internal conical 

 refraction in even feebly double-refracting crystals is the following: — 

 A polished parallel plate of crystal, cut almost at right angles to an 



