FI(>1 RE ^{. Vi»ual ucuily depi-nds upon iht- liri^htiu'»» of 

 :\n objert. This figure cftnipares llit- \i»u!il aruilie«« «if 

 >t'lloMfiii. skipjack, and littl*- 1uiin>. 



gill net; they dodged it easily. Ob.server.s .stationed at the 

 underwater ports of the Charles H. Gilbert and Toinixend 

 Cromwell have marked the apparent importance of vision in 

 feeding behavior. 



Eugene Nakamura has now measured the visual acuity 

 of three species of tuna: skipjack, yellowfin, Thiouuis 

 albucare.i (Bonnaterre), and little tunny, Euthynnus ajfinis 

 (Cantor). His experimental apparatus and methods were 

 described in the Laboratory's last progress report. Briefly, 

 the fish are taught to respond to a pattern of black-and-white 

 stripes presented on a square of illuminated opal glass. 

 Reward (food) follows one pattern, punishment (a mild 

 electric shock) another. 



The characteristic being measured is visual acuity, which 

 is one of the several elements of visual perception. Tech- 

 nically, visual acuity is "the reciprocal of the minimum 

 visible angle measured by minutes of arc." This measure of 

 visual perception depends upon the brightness of an object. 

 As the target becomes dimmer, details tend to blur. 



The yellowfin "sees better," in this sense, than the skip- 

 jack, and the skipjack better than the little tunny (figs. 2, 3, 

 and 4). Comparative size may have something to do with 

 these results, a possibility Nakamura is now investigating. 



It will be some time before these studies can be extended 

 to the other elements that sum up the visual experience: 

 the responses to color, form, size, number, position. Yet it is 

 only in such a painstaking way that quantitative studies can 

 be carried out. 



A brightly lighted square of opal glass striped black and 

 white is obviously nothing that a tuna is ever likely to 

 encounter in its natural environment. Experiments of this 

 nature represent only a first step toward an understanding 

 of the role that sight plays in the life of the tuna. Neverthe- 



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LOG OF LUMINANCE | FOOT- LAMBERTS) 



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less, it is a step that has needed taking. Equally desirable 

 have been investigations of the responses of tunas to another 

 stimulus that is also unknown in the sea — pure tones, the 

 sort of tuneless "beep" with which television stations signal 

 the passage of the hour. 



