Eyes of MoUuscs and Arthropods. 735 



stnu'tuve of the rods , wliile otlier parts of the retina, and of the eye 

 itself, were well })rcscrved: hut when allowcd to aet for lialf au hoiir, 

 at a temperature of froni 50 to 55" C, the most perfectly preserved rods 

 with their nervous uetworks were ohtained, while, on the otlier haud, 

 the remaining- tissues became so granular and homog-eneous as to be 

 nnfit for study. Chromic acid, '/io "/o) when allowed to act from oue to 

 three or four days, did not give good results, whereas, if the eyes were 

 placed in '/loVo for half au hour, in order to kill them, and theu left 

 for twenty-four hours in V2oVi) t'hromic acid, and twenty-four hours 

 more in '/lo %> ^ud fiually for forty-eight to sixty hours in •/.•i "/o, the best 

 results, as regards a good fixation of all the parts of the eye, were ob- 

 tained. 



For the cornea and general epithelium, picro-chromic acid gave 

 the best results. In such preparations, the teeth of the cornea aud 

 of the pigmeuted epithelium may be most easily seen. 



The lens is best prepared for sections by either sulphuric, or jjicro- 

 sulphuric acid ; by the first reageut, the sbape is best retained, aud the 

 leus itself is less liable to be drawn away from the surrounding tissue ; 

 the latter reageut, however, briugs out more sharply the configuration 

 of the cells, aud allows a better staiu of the nuclei to take place. 



Treatment with chromic acid, Vs'Vo ^oi' twenty-four hours, gives 

 the best result for the layer of uervous fibres just below the septum, 

 and also for the outer layer of gangliouic cells and their librous pro- 

 longations. In order, hoAvever, to study to the greatest advautage the 

 arrangement of these fibres, the retinophorae should be somewhat 

 shrunken ; this can be doue by treatiug the eyes for two or three days 

 with Y;, % chromic acid. 



The retinophorae are well preserved by nearly all the reageuts ; but 

 either in sublimate, or in picric acid, or in their combiuations, they be- 

 come slightly granular, and remain so closely packed that it is difficult 

 to distinguish the cell boundaries. Chromic acid, by which the cells 

 are somewhat shrunken, gives preparations in which the boundaries 

 and general arrangement of the retinophorae may be most easily 

 studied. 



The rods with their retinidia, and the subj acent vitreous net- 

 work, are best prepared by treatment with 1/5 Vo chromic acid at 

 50" C, for Y2 an hour. The wall of the rod remains clear and 

 glassy, but slightly tinged with yellow. The central core is less 

 refractive, aud with good light the nervous network may be easily 

 seen. I was for a long time greatly puzzled to understaud certain 



