PHYSIOLOGY: W. E. CARREY 
607 
6. Blackening corresponding halves of both eyes of Proctacanlhus, for 
example, the outer half of the right eye and the inner half of the left 
eye, causes postural changes similar to those produced by blackening 
a]] of one eye (the right) and circus motions away from the side to 
which the bJack has been applied, i.e., to the left in our example. This 
statement of fact is, however, capable of overemphasis for the intensity 
of the reaction depends upon the relative amount of black applied to 
each eye, thus if a greater area of the inner side of the left eye be black- 
ened or a lesser area on the outer (right) side of the right eye the tend- 
ency to move in a circle to' the left is lessened, nullified, or even reversed 
by combining the two procedures. This fact is an interesting and fatal 
contradiction to the view that the postures are assumed, and the circus 
motions made, in an attempt "to avoid the dark field which appears 
as an obstruction to the path." The reaction is a quantitative one and 
depends on the relative areas blackened as much as upon the part of the 
eye covered. The space relation of light fields to dark fields, does not 
change in the above experiment but the tonus of the muscles does 
change with the change in the areas blackened, whereby the behavior 
of the robber fly is reversed and it circles toward the side on which both 
eyes have been blackened. 
When either the inner halves or the outer halves of both eyes are 
blackened, the muscles of the two sides remain absolutely symmetrical 
provided the eyes are equally and symmetrically painted. If the 
painted area is considerably less on one eye than on the other, the 
tonus changes again approach those found by blackening all of one 
eye and the animal, in walking, circles to the side with the greater area 
exposed to the action of light. Complete blackening of both eyes re- 
sults in marked relaxation of all the musculature, although the two 
sides are symmetrical. 
7. All the experiments show that the muscle tone is dependent upon 
the intensity of the light and that the postures assumed depend upon the 
relative differences in the illumination of the eyes. In animals with one 
eye completely covered the radii of the circles in which they moved 
were shorter the more intense the illumination of the normal eye. With 
one eye partially covered the circles were larger than when completely 
covered and in the same way, the circles were larger when one eye was 
covered by a film of collodion or of brown shellac, which admit some 
light, than when subsequently covered by opaque asphalt black. When 
one eye was partially covered by central application of the black paint 
the tilting and circling to the opposite side were abolished or reversed 
by brilliant illumination of the partially blackened eye. These re- 
