HOMING OF SEA-SWALLOWS 5 
and out past Tortugas through the Straits of Flor- 
ida. The current differs in color from the surround- 
ing water and from the return current which runs 
nearer the coast-line. But the color difference is 
only noticeable when the sun is in a certain position 
in relation to the observer; many of the successful 
birds were liberated at night, and all were out for 
several nights; they had to win their way home 
through rain, haze, and cloudy weather; they 
homed equally well, no matter at what point between 
Galveston and the Tortugas they were put down. 
And besides, why should they not follow the current 
in the opposite direction? (3) It has been sug- 
gested that the birds get their bearings visually 
by ascending to a great height. But, in the first 
place, they never seem to rise very high; in the 
second place, they would require at a distance of 100 
miles to ascend almost a mile to see the Loggerhead 
Key lighthouse; and, in the third place, even if 
they ascended they would not see much because uf 
the continuous haze. 
The observers are not inclined to assume any new 
and mysterious ‘“‘sense of direction” untii they 
have made many more experiments, and a good 
beginning has been made. Thus, to meet Duchatel’s 
hypothesis that the retina of the bird is specially 
sensitive to infra-luminous rays, especially infra- 
red, Professor Watson made a special investigation 
of spectral sensibility in the chick and the homing 
pigeon, and found no evidence at all of the sup- 
posed susceptibility. Care was also taken to test 
