400 Intelli(jence and Miscellaneous Articles. 



the objection that in this way a perfectly pure spectrum is with 

 difficulty obtained. It almost needs a lucky accident for the posi- 

 tion of the lenses and the accommodation of the eyes to cooperate 

 so favourably that the eye observed sees distinctly the Eraunhofer's 

 lines at the same instant in which the observer has the background 

 of the eye sharply before him. Now, although a slight deviation 

 from perfect purity cannot do much harm when the blood-vessels 

 are made use of as a criterion of fluorescence, and consequently the 

 mere superficial judgment of the observer is enough for the attain- 

 ment of sufficient purity, yet it was still desirable to entirely 

 obviate the objection mentioned. This was done in the following 

 manner : — - 



By means of a large, powerfully dispersing, direct- vision prism, 

 a spectrum was projected of 40-50 centims. length. The observer 

 now repaired to the region of the image-plane, and caught with 

 the speculum now this, now that sort of rays, so that the observed 

 eye was successively illuminated with nearly monochromatic light 

 of A^arious degrees of refraugibility. 



The colour of the vessels appeared every time exactly as it did 

 with the method above described — and, indeed, not only with tlie 

 brown rabbit usually made use of for the experiments, but also 

 with man. 



This mode of investigating is much more convenient than the 

 above-described ; but it involves the impossibility of seeing all the 

 phenomena at one view. 



At first all the experiments were made with lenses and prisms 

 of glass, and parth^ indeed, using only a skioptilvon as the source 

 of light. It now appeared desirable to repeat them with quartz 

 lenses and prisms, when, of course, sunlight was employed ; but 

 herein it was not possible to use the second mode of observation, 

 since the spectrum could not be rendered sufficiently large. On 

 this account we were compelled alw^ays to project a somewhat 

 larger portion of the spectrum upon the retina. 



The results were in general the same as those above described ; 

 only the brown-red colouring of the blood-vessels came out more 

 vividly than when glass apparatus was employed (the speculum, 

 however, and the lenses belonging to it were still of glass) ; and, 

 finally, it was in this case possible to trace the phenomenon as far 

 as into the ultra-violet. After the entire visible spectrum ivas inter- 

 cepted, heing graducdly brought to vanishing behind the pupillary 

 margin, so that only ultra-violet rays fell upon the retina, the vessels 

 of the retina appeared in a decidedly reddish tone upon a dirty grey- 

 broiun ground. 



To sum up, by these observations it appears to us demonstrated 

 that the living retina also does actually fluoresce, and under the influ- 

 ence of the same rays which Helmlioltz had already recognized as 

 exciting fluorescence in the dead retina. 



As a mark of fluorescence the colour of the retina- vessels served, 

 which in blue, violet, and ultra-violet light approached more and 

 more to the natural colour of the blood — a fact only to be accounted 

 for by fluorescence of the layers situated behind them. — Berichte d. 

 baier. Akad., maih.-phys. Classe, July 7, 1877. 



