270 Mr. W. G. Horner on the Vascular Spectrum. 



themselves, are situate rather more internally than the me- 

 dium and smaller vessels. And this supposition derives sup- 

 port from the fact that the trunks of the upper and lower 

 branches, when narrowly observed, have a perceptibly greater 

 motion than the general system has (Art. 10.) ; the excess 

 being distinguished by an alternating movement, like that of 

 a pair of shears, gently pressed and released by turns ; — the 

 perspective reason of which effect is very obvious. The esti- 

 mated magnitude of the medium vessels also requires correc- 

 tion (Art. 11.). On comparing their shadows with a gradu- 

 ated system of bars at 15 feet distance, f inch proves more 

 correct than ^ 5 and this element introduced into the corrected 

 calculation gives ^ n inch, or less, for the diameters of the 

 larger vessels in fig. 2. Of these, and yet more delicate ves- 

 sels, it is almost needless to remark, that not a trace appears 

 in Sbmmerring's designs; at least, in Schroter's copies of 

 them. Nor is it, perhaps, to be expected that they can be 

 anatomically demonstrated. 



The appearance of the granulous spot in the experiment 

 above detailed is interesting and splendid, occupying, at 16 

 or 17 feet distance, an area of a foot or more in diameter, and 

 strongly resembling in its entire description the pyrotechnical 

 exhibition of what is termed golden rain, when just beginning 

 to descend. 



The detail of these vessels must differ greatly with different 

 observers, since a remarkable difference is found between 

 those of my own right and left eye, the latter being more 

 regularly distributed, and those of the upper and lower 

 branches more decidedly intermingling their extreme and 

 minute vessels. Yet the resemblance of the general structure 

 to that exhibited in two of Sommerring's figures, as Sir D. 

 Brewster has already remarked, is too striking to admit a 

 doubt to rest on the identity of the subjects. In fact, Zinn's 

 description of the " arteries of the retina " is an accurate de- 

 scription of the very view of things to which my experiments 

 have conducted by an independent route : " Arteriolar retina? 

 unice in interiori retina? facie decurrunt, nudae fere, qua hu- 

 morem vitreum contingunt ; et teguntur, qua choroidem re- 

 spiciunt, multa medulla nervi optici." (cap. x. § 4.) 



The reflection that has most struck me, in reading, for the 

 first time,this author's elaborate and beautiful description of the 

 retina, its double structure, vascular and medullary, in distinct 

 and parallel strata, and its by no means evanescent thickness, 

 is this, that it is very surprising that so many writers who have 

 recognised the retina as the seat of vision should have treated 

 it as a mere geometrical superficies, or else have conceived of 



