576 HOWARD. [Vor. XIX. 
IT. Osmic Acid Material. 
Osmic acid, or osmium tetroxide, (OsO,) has held probably 
as important a place as any fixing reagent in the investigation 
of the visual cells (Schultze, 66-72: Hoffmann, ’75; Greef, 
7 OO): 
I have used osmic acid without admixture of other reagents 
chiefly in the form of vapor. Eyes which were not opened 
were suspended over a 2 per cent. aqueous solution of osmic 
acid in a tightly closed bottle for three minutes. This method 
seems to have distinct advantages over the use of this reagent 
in fluid form. It avoids the danger of distortion and tearing 
of cells by osmotic pressure; as no opening of the eye is 
necessary, the danger of disturbance from other mechanical 
causes 1s also eliminated. Penetration is very rapid, two 
or three minutes’ exposure being sufficient to fix the rods 
and cones. <A retina thus fixed may be teased immediately 
upon a slide in glycerine, and isolated rods and cones found 
in the field of the microscope (Pl. 4, Fig. 33); or the whole 
eye, after it has been hardened in alcohol and its lens has been 
removed, may be cut into sections and mounted in balsam 
(Pl. 4, Fig. 35). Dyes do not readily stain tissues which 
have been treated with osmic acid; at least, I have been 
unable to get evidence of fine differentiation even after bleach- 
ing, and treatment with potassium bichromate as recommended 
by Lee (:05). If preparations fixed in this way were more 
susceptible to stains, the method would furnish, no doubt, 
a most valuable check on other methods of fixation. 
In a retina from Necturus fixed as described by osmic acid 
fumes, the rods and cones have the following appearance. 
The cylindrical outer segments of the rods (Pl. 4, Fig. 33, 
prs. dst.) vary in tint from black to a translucent brown, 
depending upon the action of the fixing fluid, whether over a 
longer or shorter period. The other portions of the rod, as well 
as the whole of the cone, show varying shades of light brown 
or greenish grey, of much less striking character. 
