198 ADAPTATIONS TO DIURNAL ACTIVITY 



as compared with a compromise yellow one. On less bright days, the tur- 

 tle's yellow, or even his colorless, droplets automatically replace the red 

 ones as the most important constituents of the whole mosiac. Thus the 

 birds and turtles, having sufficiently cone-rich retinae, have been able to 

 differentiate the cones into several populations. Where most retinae are 

 rod-and-cone, or duplex, the turtles and birds have produced what may 

 fairly be called multiplex vision. 



The workings of the turtle's oil-droplet mosaic can best be gathered 

 from an account of a clumsy, man-made imitation worked out empirically 

 by the United States Navy, as described to the author by Mr. Laurence 

 Radford of the Bureau of Ordnance : 



"The Navy uses both red and yellow color filters in optical instru- 

 ments. Both are made of Corning Glass. The red cuts quite sharply at 

 about ^6000-6200 [A.u.j and the yellow at about ?u5 100-5300 [A.u.]. 

 These filters are used because much experience has shown that they are 

 helpful, and the particular filters selected were chosen after considerable 

 study, both experimental and theoretical. 



"In my opinion these filters are effective for our purposes because they 

 reduce glare due to scattered light and minimize the eflFects of the chro- 

 matic aberration of the eye, and for these reasons almost exclusively. 

 These two effects are produced more intensively by red than by yellow 

 filters, i. e., the amount of scattered light transmitted by the red is much 

 less than by the yellow because the latter transmits the green; and with 

 the red filter the effect of chromatic aberration is practically eliminated. 

 But there are conditions when the red filter cannot be used effectively, 

 perhaps because of insufficient intensity of illumination, or perhaps be- 

 cause it would reduce the color contrast. Hence the two colors, giving us 

 essentially the choice of two degrees of the same effect." 



The red and yellow oil-droplets of the domestic hen have been found 

 to cut the spectrum off respectively at ^5800-5900 A.u. and ^5150-5250 

 A.U., the extracted red pigment (astacin) and yellow pigment (xantho- 

 phyll) at A,5900A.u. and ^5200 A.u. respectively when dissolved in 

 castor oil. Perhaps when the droplets of turtles are studied more carefully 

 they will be found to come even closer to justifying the Navy's choices! 



In this connection it is significant that the kingfisher, whose visual 

 problem, like that of the turtle, is complicated by glary water, has 60% 

 red droplets — three times as many as the average bird. So much for the 

 functions of the intra-ocular color-filters. Some remarks on their nature 

 and evolution are now in order. 



