398 Intelligence and Miscellaneous Articles. 



which are conditioned partly by the absorption-phenomena of the 

 blood, and in part by fluorescence. The aspect of this pictm^e is 

 extremely beautiful. Awhile the retinal background shows the 

 picture of the spectrum in the usual manner, the vessels stand out 

 in very various colouring : shown upon the red background only 

 by a somewhat intenser tint, in the orange almost absolutely un- 

 recognizable, they appear in the green deep black in wonderfully 

 sharp outline, but in the blue in a dark brown-yellow tint, which 

 becomes in the violet a dark red-brown. In the orange they are 

 so feebly prominent that one might think they were filled with 

 water, while, on the contrary, at the commencement of the green 

 they become suddenly as black as if they contained ink ; and this 

 leap from the one into the other tint takes place in such a striking 

 manner that they appear almost as if cut off. As is well know^n, 

 in feebly luminous spectra pure yellow is not perceptible ; we 

 know, however, from experiments on the absorption of blood that 

 it commences in the region of Eraunhofer's line D, and, indeed, a 

 little preceding it, as follows from the fact that the vessels when 

 illuminated with the light of sodium are likewise seen black. 



Accordingly, as far as into the green the phenomenon described 

 corresponds closely to what might have been expected from the 

 absorption-spectrum of blood. 



In the blue, however, in the region of the line E, things are 

 changed. If we had here to do with absorption-phenomena only, 

 the vessels would of necessity be marked similarly as in the red — 

 that is, merely by a difference of brightness — although somewhat 

 more sharply ; we might then expect merely darker blue vessels 

 upon a moderately bright ground. Instead of this, they appear in 

 a yellowish broM^n tone, reminding one of very dark ochre, while 

 in the violet they show a decided tendency to red, somewhat like 

 rusty iron. 



This phenomenon can only be explained by fluorescence of the 

 underlying retina ; and thus the vessels of the retina furnish the 

 finest test of the presence of fluorescence. 



At the first instant, one might indeed think that the phenomenon 

 was one of contrast ; but a somewhat closer consideration teaches 

 us that this supposition is here inadmissible. Eor, in the first 

 place, one could not perceive why the phenomenon should be absent 

 from the green, which yet we know to be specially apt to call forth 

 contrast-phenomena ; and then the colour of the vessels would 

 necessarily approach yellow-green from the blue to the violet, 

 while, on the contrary, the dark brown of the vessels passes over 

 into red-brown in this region of the spectrum. 



Moreover fluorescence of the blood itself cannot be the cause, 

 since we at least have not succeeded in discovering in blood a trace 

 of fluorescence ; besides, it viould be extremely improbable that, 

 in transmitted light, red blood should show red fluorescence. 



Just as little can the cause be sought in diffused light that in 

 some way has penetrated into the eye, or, in any case, find its origin 

 in the fluorescence of the cornea or the lens ; for such light must 

 show its action equally in the green portion of the spectrum- — nay, 

 there in increased measure, since the contrast must sharpen the 



