802 M. Treviranus on the Organic Structure 



necessary to select a clear day, when having taken small bits of 

 the retina of an animal recently killed, and cut oflp with an ob- 

 lique edge, unmoistened by water, and uncleansed from the par- 

 ticles of vitreous humour that adhere to their surface, you must 

 place them on the object-glass of a compound microscope, which 

 magnifies SOO times, gives an image of great clearness, and 

 possesses a well illuminated field of vision. On the outer sur- 

 face of these bits of retina, one may distinguish the radiated 

 course of the medullary cylinders, and on their slanting edges 

 may be seen the inward progress of the same, together with the 

 regularly arranged, though crowded papillae which cover the 

 inner surface. The vascular network and the sheaths of the 

 papillae cannot be accurately observed on such fresh bits of re- 

 tina, and consequently, to distinguish these, a particular mode 

 of dissecting the retina, previously hardened by spirit of wine, 

 is recommended by Treviranus. He gives the following mea- 

 surements of the semidiameters of the papillae in different ani- 

 mals expressed in decimals of a Paris line : 



These semidiameters, observes Treviranus, obtained by means 

 of the most accurate micrometrical measurement, are in general 

 several ten thousandth parts greater than the number which I 

 have already proved to represent the maximum which the ra- 

 dius of the image of a luminous point on the retina must pos- 

 sess, in order to excite the sensation of one indivisible object. 

 Now we cannot with certainty affirm it to be requisite for clear 

 and distinct vision, that the concentrated rays from such a point 

 should strike the retina only on one papilla ; such, however, is 

 probably the fact. 



Each papilla is convex, and undoubtedly most sensibly af- 

 fected, when the middle of the image, or that part where all the 

 rays meet, coincides with the centre of the papillary elevation ; 



