1896.] Acute Vision in the Vertebrates. 27 



replaced with fresh by removing the stopper in the lower part 

 of the eye. After remaining twenty-four hours in each of the 

 following liquids : 80, 90, 95 per cent., absolute alcohol and 

 absolute ether (1 part each), it is then changed to celloidin. 

 Best results are obtained when three grades of celloidin are 

 used— 1st, very dilute ; 2d, less dilute ; 3d, as thick as will run. 

 It is allowed to remain from four to six days in the first, six to 

 eight days in the second, and ten to fifteen days in the third. 

 If the eye is kept well under pressure throughout this process, 

 the retina will be well preserved and lie smoothly against 

 the choroid. 



I have tried other liquids for hardening the eye whole, but 

 with poor success. Have tried the method of Barrett and of 

 Cuccati, but, in each case, the retina was very much wrinkled 

 and folded, while the whole eye was much shrunken and out 

 of shape. In vapors of osmium, I have had fairly good results 

 with the retina, but the same trouble, due to the shrinking of the 

 whole eye, is present. Chievitz says 2 that a fish's eye may be 

 preserved whole, with retina lying nicely back, by simply im- 

 mersing it, or even the whole head, in 80 per cent, alcohol. 

 The hardening agent which he generally uses is 2.5 per cent. 



Another method which I have employed with small ani- 

 mals, especially birds, in order to demonstrate quickly the 

 presence or absence of a fovea, is to immerse the whole head 

 in Perenyi's fluid for from three to five hours. This will 

 harden the eyes so that the cornea, lens and vitreous humor 

 may be removed, leaving the posterior half in situ. With 

 birds I have had good results, the retina lying back smoothly so 

 that the fovea and entrance of the nerve, marked by the pec- 

 ten, may be easily seen. Fig. 3 represents diagrammatically 

 the appearance of the retina after the front of the eye has been 

 removed. 



In order to show the angles which the lines of vision make 

 with the median plane, sections were made through the whole 

 head of several animals (fish, amphibians, reptiles, birds and 



Anatonrie and Entwickelungsgeschichte, Sup., Band, 1889, p. 141-142. 



