114 Mr. G. H. Fielding on a 7iexv Memhrajie of the Eye, 



Hunter, Fyfe, and others seem to do), we shall have two dif- 

 ferent things occupying the same place, which is impossible. 

 Other anatomists* regard Tapetum and Membrana Ruys- 

 chiana as synonymes ; and, as Sir Charles Bell justly observes, 

 considerable confusion prevails I'especting the precise mean- 

 ing of the term Tapetum. He, however, defines it to be a pile 

 or fleece laid upon the Membrana Ruyschiana, and states the 

 pigment to be spread upon the tapetum next to the retina, and 

 consequently between it and the retina. It is useless, however, 

 disputing about terms ; for all anatomists seem agreed that 

 the pigment is placed immediately behind, and in contact with 

 the retina; and as the part of which I am treating is immedi- 

 ately behind, and in contact with the retina, I apprehend that 

 the following proposition will be the only one I shall have to 

 prove, viz. that the substance placed immediately behind, and 

 in contact with the retina, and known by the name of Pig- 

 mentum nigrum, is a membrane, and not a pigment. 



1st, We have already seen that it does not stain paper, and 

 is not removeable by washing. — 2ndly, Take a section of an 

 eye in which the colours are vivid, place it on the table in a 

 bright light, and fixing your eye on any part, steadily, walk 

 round the table, you will find the colour varies according to 

 the different positions you view it in. — 3dly,It presents a bright 

 polished surface, like that of a well-finished mahogany table. — 

 4thly, Carefully detach a small portion of this substance, and 

 put it between two thin pieces of glass ; it will present a hard 

 and well-defined outline, and on putting the glasses in closer 

 approximation and suddenly relaxing them, you may perceive 

 the substance expand and contract. Again, view this portion 

 by reflected light, you will perceive its usual colour, but with 

 transmitted light you will have a totally different one. In this 

 point it follows Sir Isaac Newton's laws as regards the colours 

 of thin plates. — 5thly, I detached very carefully a small por- 

 tion of this substance, and placing it between two pieces of fine 

 thin glass subjected it to examination through a fine achro- 

 matic Amician microscope by Chevalier; the colour of the 

 portion thus examined, was pale blue by reflected, and red- 

 dish-yellow by transmitted light. When placed in the field of 

 the mici'oscope the same change of appearance was observed 

 to take place on viewing it as an opake and as a transparent 

 object. With a power of 800 to the diameter, not only were 

 blood-vessels apparent, but even the globules in those vessels ! 

 and by increasing the magnifying power to its utmost extent, 

 the globules appeared of the size of a very small pin's head. 

 I once thought 1 could trace nervous filaments, but do not 



* Shaw and others. 



i 



