74 



Messrs. Jones and Dupre on a Substance, [Apr. 12, 



short time, and the urine sometimes, are the best-known fluorescing 

 substances. 



But as long since as 1845, Professor Briicke, in Miiller's * Archiv,' stated 

 that he had found in many and very frequently repeated experiments, that 

 the lens absorbed the blue rays of light to a very great extent, and that the 

 cornea and the aqueous humour did so to a less extent, but that the lens 

 together with these other media absorbed these rays to the greatest de- 

 gree. He used the eyes of oxen and of rabbits, and the lens of a pike's 

 eye, which last, when dried with care, preserved its transparency, and allowed 

 the light to fall on a porcelain plate covered with tincture of guaiacum and 

 bleach a portion of the green surface. 



Professor Stokes, in his well-known paper " On the Change of the Re- 

 frangibility of Light," in the Philosophical Transactions for 1852, says 

 (p. 512), "It is found that the property of change of refrangibility in the 

 incident light is extremely common." " To make a list of sensitive sub- 

 stances would be useless work ; for it is very rare to meet with a white or 

 light-coloured organic substance which is not more or less sensitive." 

 Among others he mentions horn, bone, ivory, white shells, leather, quills, 

 white feathers, white bristles, the skin of the hand, and the nails. And in 

 his conclusion (p. 557), he says, " The phenomenon of change of refrangi- 

 bility proves to be extremely common, especially in the case of organic 

 substances, such as those ordinarily met with, in which it is almost always 

 manifested to a greater or less degree." 



When speaking of the fluorescence of sulphate of quinine, he says 

 (p. 541), "When quinine was dissolved in dilute hydrochloric acid, the 

 blue colour was not exhibited, not even when the fluid was held in the 

 sunlight and examined by superficial projection." 



In 1855 Helmholtz published a paper in PoggendorfF's 'Annalen,' vol.xciv, 

 p. 205, in which he says that, as far as quinine paper showed that the 

 spectrum extended, so far the eye could perceive light *. 



He then proposes this question. Does the retina see the rays beyond the 

 violet directly as it sees other colours of the spectrum, or does it fluoresce 

 under the influence of these rays ? and is the blue colour of the rays beyond 

 the violet light of less refrangibility which shows itself in the retina only 

 imder the influence of the violet rays ? 



To determine this question, he says, I examined for fluorescence the 

 retina of the eye of a man who had been dead for eighteen hours. The first 

 experiment showed that it was very feebly fluorescent. The retina was 

 less fluorescent than paper, linen, and ivory, but more than porcelain. 



In 1853 Prof. Donders, in a paper in Miiller's ' Archiv,' p. 471, " On the Action of 

 the Invisible Eays of high Eefrangibilitj on the Media of the Eye," says that " most, 

 if not all, the rays of higher refrangibility than the violet reach the retina, and are not 

 absorbed by the different media through which the light passes ;" and Dr. Kessler, in 

 ' Archiv filr Ophthalmologic,' 1854, vol. I. p. 449, says that "the crystalline lens is not 

 the cause of the invisibility of the rays of high refrangibility ; for when the lens is 

 removed by operation these rays are not more visible. 



