No. 3.] VISUAL CELLS IN. VERTEBRATES. 575 
ing was done on the slide by the potassium chlorate method 
(Lee, :05), or by the potassium permanganate and oxalic 
acid process (Mallory :05). 
In the study of fresh material the cornea and lens were 
carefully removed, the eye divided, and the retina placed on 
a slide. In this process as much of the eye fluid as possible 
was removed with the retina, which was then teased in it 
and covered. The fluid at the edge of the coverglass coagulates 
and promptly checks further drying from exposure to air. 
With such a preparation, a field containing detached rods 
can readily be found. These are usually broken, but occa- 
sionally a whole element was found intact, as for instance 
where a fortunate fold of the retina gives the visual cells in 
profile. 
When more fluid than could be obtained from a single eye 
of Necturus was needed, humor from the eye of a frog or an 
artificial examination medium was used. 
In tests for suitable media the most satisfactory that was 
found was the more fluid portion of white of a hen’s egg. 
In the more artificial media that I used, the elements disinteg- 
rate very rapidly. 
Fresh preparations, put up as described were studied either 
under an ordinary microscope or a polarizing ne. 
The effects of reagents and stains were determined by 
introducing solutions of these (usually very dilute) at the 
edge of the cover-glass, while the object was under obser- 
vation. 
B. PERMANENT PREPARATIONS. 
Observations of the visual elements begun on fresh unfixed 
material would be a logical introduction to their study, but, 
because of the difficulties in manipulation and interpretation, 
I found it hardly practicable. One must first gain some 
familiarity with the objects from a study of sections or, at 
least, from hardened material. On this account I have under- 
taken to describe first permanent preparations, selecting those 
which gave evidence of best preservation. 
