49 
stimuli. One of the main problems of this method is to devise a 
technique that minimizes the chief sources of error. 
As is well known, the frog’s pupil is an elliptical-like opening the 
long axis of which is almost parallel to the long axis of the body. 
On the lower margin of the pupil, almost in line with the short axis 
of the ellipse, there is a small notch, whereas the upper margin, 
though usually continuous, is sometimes indented by a less con- 
spicuous notch. In measuring the eye, especially when widely 
dilated, these notches furnish convenient points of reference in locat- 
ing the short axis. The long axis in the undilated eye is of course 
easily located by reason of the shape of the anterior and posterior 
margins of the pupil. 
In orienting the bulbus previous to measuring the size of the pupil 
it is necessary to have a suitable container. Glass vessels of about 
3 c. c. capacity were used, made from hard bacteriological test tubes 
17 mm. in diameter, the edges being ground smooth so that it might 
be sealed with a cover slip to avoid evaporation. 
Fig. 1.— Water bath, a, Glass container, described in text; b, water bath through which a steady- 
stream of water from the temperature regulator is kept flowing. 
Except in a few experiments described later, special care was 
taken to avoid the changes in temperature and light values that 
occur in a laboratory lighted by direct or diffuse sunlight. To this 
end a dark room was fitted up with a 16-candlepower incandescent 
light so arranged that the excised bulbi could be lighted from a given 
distance, thus making it possible to maintain a light of constant 
intensity. Of course at a distance of 10 inches such a light generates 
enough heat to warm 2 c. c. of solution to 30° or more, depending 
upon the room temperature. In order to avoid not only such high 
temperatures, but more especially fluctuations in the same, the 
water bath, indicated in figure 1, was devised. It is constructed in 
such a way that the glass container rests in a current of water, the 
temperature of which is so regulated that the temperature of the solu- 
tion in the container can also be kept constant. 
Preliminary experiments demonstrated that a mere naked eye 
observation of the changes in size of the pupil is of little value, and 
so an instrument of precision was sought that would enable the 
177— Bull. 55—09 4 
