InteUiyeyice and Miscellaneous Articles. 399 



impression*. Moreover fluorescence-light issuing from parts of 

 the eye situated in front of the A^essels of the retina could not but 

 make itself perceptible, even in viewing Purkinje's figure in mono- 

 chromatic illumination — which, according to our observations, is 

 not the case. 



The peculiar brown and red-brown colouring of the retina-vessels 

 in the blue and violet parts of the spectrum, can consequently be 

 explained only by fluorescence of the retina situated behind them ; 

 indeed the commencement of it must be sought in the same place 

 in which it was found by Helmholtz in the dead retina. 



Although, according to what has just been said, the fluorescence 

 of the living retina has already been proved by this one experiment, 

 yet we were not willing to rest satisfied with this, but to pursue 

 the matter still further in different directions, in order thus to 

 procure further material of proof. In the first place we endeavoured 

 to imitate the phenomena seen upon the retina ; for we brought a 

 capillary tube filled with blood-solution before a spectrum projected 

 upon a surface. Here also exactly the same phenomena were seen 

 in the red, orange, yellow, and green, in the blue and violet like- 

 wise when the screen fluoresced in these colours. It is true that 

 here the slightest trace of fluorescence is perceived, so that even 

 white paper permits it to be distinctly recognized ; and the colour 

 of the tube before such a screen reminded one, in all parts, of the 

 blood-vessels on the retina ; that here, however, we had in fact 'to 

 deal with the light of fluorescence which issued from the screen 

 was strikingly shown by this — that the brown and reddish colouring 

 of the capillary tube totally vanished when the screen was removed 

 so that the black space of the dark room served as background, and 

 that it came up much more decidedly when for the white paper 

 screen one coated with platinicyanide of barium or other fluorescent 

 substances was substituted. These experiments convinced us 

 that, in a transparent coloured substance held before another body 

 serving as projection-surface for a spectrum, we possess an ex- 

 tremely sensitive means for the recognition of traces of fluorescence. 

 Whenever this test substance, in a part of the spectrum in which 

 it when alone does not permit its natural colour to be recognized, 

 shows it if there is a body behind it, this must be the result of 

 fluorescence-light emitted from the latter body. 



Shadow- streaks were also called forth in a projected spectrum, 

 in order to distinguish with complete certainty the influence which 

 contrast-phenomena might possibly in any case exert upon the 

 observation. The conclusion was, that the colour of the retina- 

 vessels observed in the blue and the violet is in no case the con- 

 sequence of phenomena of contrast. 



After settling this more physical side of the matter, the question 

 now was, to further extend the ophthalmoscopic investigations and 

 vary them in divers ways ; for the method above-mentioned, in 

 which the entire fan of coloured rays is taken up by the speculum 

 and a minute spectrum is thrown upon the retina, is not free from 



* With very intense illumination this kind of light actually does appear 

 to assert its influence ; and on this account moderate illumination of the 

 retina is to be preferred. 



