318 
experiments, that cone summation 
must occur at some level, for the 
threshold for brightness discrimina- 
tion (at high illuminations as well as 
at low) is higher the smaller the area 
of the retina stimulated. Now the 
mean illumination of the retina when 
the eye is looking at a bright sky is 
not changed by movement of a dark 
Parus major 
yap 
canis dence canis dence 
- Archibuteo lagopus Falco subbuteo 
Ficure 7.—A selection of tracings of the pecten shadow upon the retina. 
object, such as a swift, within the 
inspected area provided the optical 
path is unobstructed. If, however, a 
series of shadows is cast on the retina 
by an obstruction, it is clear that the 
mean illumination will be a maximum 
when the image of the object coincides 
with one of the shadows. Conse- 
quently movement of the object across 
the field will cause successive rises and 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
falls in the mean illumination. It is 
therefore quite possible that the pec- 
ten is effective in increasing contrast, 
but it is doubtful, perhaps, whether 
it is proper to speak of the effect as 
enhancing perception of movement. 
The contrast is improved equally 
whether it is the object or the eye 
which moves, and the improvement 
Strix flammea 
Buteo buteo 
Falco peregrinus 
(From Menner, 
is likely to be most marked when the 
background is so neutral and feature- 
less that there is no fixation point to 
keep the eye still. 
NOCTURNALITY 
Nocturnal birds have eyes special- 
ized for night vision, though to a less 
extent than those of completely noc- 
