232 HENRY LAURENS AND S. R. DETWILER 
smaller member of the double cones shows an average change 
in length of 3.5m, the larger member of 2,7^ (figs. 7, 8 and table 1). 
5. The actual change in position of the pigment between hght 
and dark eyes is shght, averaging but 1.6^; but when combined 
with the change in length of the visual cells, gives an, effective 
migration equal to the sum of the two (figs. 9, 10, 11). 
6. A theoretical consideration of photomechanical changes 
and of the duphcity theory from a comparative point of view 
is appended. 
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