114 ISatural History Bulletin. 



miss bagging any of them unless he picks out a particular bird 

 and aims directly at that. It is probable that the rapid circling 

 of the birds over Bird Kev had the effect of creating- an 

 optical illusion, whereby their number is greatly multiplied. 

 Again, the eyes not being focused on any particular individual, 

 the apparent number is double the actual number. A very 

 simple experiment will illustrate this principle. Take a white 

 sheet of paper and make a number of perforations in a group 

 with the point of a penknife; then hold the paper between 

 your eyes and the light. If the sight is focused directly on 

 the holes, their apparent number will be twenty. If, how- 

 ever, the sight is directed in a general way toward the surface 

 of the paper, without reference to the perforations, there will 

 presently appear two instead of one group of holes. In other 

 words, their number will apparently be doubled. It is thus 

 evident that the number of flying birds is multiplied first by 

 the optical illusion caused by the motion, and again by another 

 optical illusion caused by the sight not being focused. 



On Bird Key we found two or three graves of sailors who 

 had been buried in this lonely spot. There was something 

 peculiarly desolate in the surroundings. The glare of the 

 sua on the white coral sand, the swish of the wind through 

 the low scraggy vegetation, the rustle of the grotesque land- 

 crabs as they scurry away in the grass, and the screaming of 

 the circling gulls and terns, convey an idea of dreariness, 

 intensified by a lack of repose not in harmony w ith the rude 

 graves with their weather-worn head-boards. 



A short distance to the east of Garden Key are a couple of 

 small, bare islets called Garden Ke\- Reefs, while Bush Key 

 lies, almost bare of vegetation, several miles to the northeast. 

 East Key, the onl}- other of the group visited by us, lies almost 

 directly east of Bush Key, and is barely visible from the deck 

 of a vessel at the fumigating dock. There seem to be no 

 indigenous trees on any of the islands forming the Tortugas 

 group. Quite a number of cocoanut palms and other useful 

 trees attain a yery satisfactory growth on Garden Key and 

 Loggerhead Key. The islands seem to be composed entirely 



