Intelligence and Miscellaneous Articles. 445 



were afterwards slowly modified; and ten hours after the forma- 

 tion of the flower some small black spots were distinguished upon 

 the metals. The observation had to be discontinued on account of 

 the lateness of the hour; and the next morning several of the 

 petals had burst. 



I ought to mention that the glycerine liquid which was used for 

 this experiment was of very indifferent quality, perhaps because 

 the ole?„te employed was not prepared with the requisite care, or 

 because Price's glycerine is nowadays not so pure as it was for- 

 merly. But this may have been an advantage ; for with an excel- 

 lent liquid the modifications of the flower would have been too slow. 



If after withdrawing the flower from the liquid, and while all the 

 petals are still colourless, they be successively burst in the middle 

 by a wire heated in the flame of a spirit-lamp, a numerous series of 

 brilliant little liquid masses are instantly seen attached to the 

 entire metallic contour, which threads them like the beads of a 

 necklace : their arrangement is very nearly regular ; and there are 

 fourteen of them to each petal. 



I seize this occasion to insist on two precautions to be taken in 

 making the oleate of soda, precautions indispensable if we wish to 

 form good glycerine liquid. In the first place, the oleic acid ob- 

 tained at one stage of the preparation must be most carefully pro- 

 tected from the action of the oxygen of the air ; in the second 

 place, the sea-salt employed to separate the oleate at the end of 

 the operations must have been perfectly purified. Let us add that, 

 when the oleate is prepared, we must take care not to dissolve it 

 in alcohol to render it more pure ; I believe I have proved that, on 

 the contrary, this spoils it. 



Let us, in conclusion, with respect to the experiment of the 

 flower, mention a curious instance of persistence of impressions, 

 an instance moreover which is not unprecedented*. My wife, 

 whose eyes are very sharp, had observed the flower at different 

 times during the day. Now, at her waking on the morrow, casting 

 her eyes upon the white curtains of the bed, she saw the image of 

 the flower : one of the petals was so clearly delineated that she 

 distinguished even the wire forming its outline ; the interior was 

 striped with white and yellow ; the other petals were more or less 

 indistinct. Changing the direction of her gaze caused the image 

 to disappear, — Eoctrait des Bulletins de V Academic Roycde cle Bel- 

 ffique, ser. 3, t. ii. no. 7 (1881). 



PHOTOMETRIC COMPARISON OF SOURCES OF LIGHT OF DIFFERENT 

 TINTS. BY A. CROVA. 

 The exact measurement of the ratio of the intensities of two 

 lights of which the emission-temperatures are very different pre- 

 sents great difficulties, on account of the difference of composition 

 of the two lights which are to be compared. The equality of inten- 

 sity of either the shadows or the illuminated regions of the photo- 

 metric screen cannot be exactly determined, because of the differences 



* See Stevelley, LInstitut, 1859, no. 1309, p. 38; Uoiiiii, Ann. di 

 Ottalm. t. iii. p. 164. 



Phil. Mag. S. 5. Vol. 12. No. 77. Dec. 1881. *2 L 



