834 W. LeConte Stevens— Physiological Optics. 



in order to produce a concave or convex spectral binocular 

 image. His eyes did not have enough muscular power to test 

 the effect of varying the tension in either ciliary or rectus mus- 

 cles, nor was he able to perceive duplication in any part of any 

 binocular spectral image. 



5. A pair of diagrams were constructed in such manner as to 

 show very plainly the binocular duplication of central parts ill 

 the background when the foreground was binocularly regarded, 

 and the gaze was monocularly directed to the center of each in 

 succession, with the usual precautions. The spectral image 

 presented the appearance of relief. By an effort of special 

 attention the duplication of the background became percepti- 

 ble, but at the same moment the appearance of relief was lost. 



These experiments, combined with those made under the 

 light of the electric spark, show very conclusively that the 

 from being conducive 

 to clearness of binocular perception, tends rather to interfere 

 with it. If it be said that we uuconsriously perceive them and 

 distinguish between the two kinds, this conclusion cannot be 

 continued or disproven. except so far as experiments like those 

 just, detailed may be accepted as having some bearing upon the 

 subject My own disposition is to discard intuition entirely, 

 and. with Ileiniholtz, to regard the degree of attention bestowed 

 upon objects pictured at the same moment on different parts of 

 the two retinas as an element of more importance than cither 

 plav of the eves or the perception of double images. The point 

 in the field o"f view to which most attention is habitually given 

 is that pictured upon corresponding retinal parts; but the at- 



eacti eye. rue mental suggestion aue i 

 n-corresponding parts is that of the thir 

 If this be called the perception of doubl 

 ems to be dependent upon their not emer^ 

 " dd to this the fact that thegradatio 

 • vision is whollv imperceptible, an 



i experience is our habitual guide, t 

 lys a reliable one. Whether intuitio 

 ) additional guide at all, it is not easy 1 



Tic 



: debate betw< 



?en the advocate 



s of the ei 



ut'.'iri 



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istit 



s theories is do 



ubtless like the 



well kno' 



.vn qi 



jarrel about a 





ain shield, and 



i may be con tint 



led indefin 



it.-l.v. 



The domain 



of i 



ntuition is ce: 



rtainly more lin 



dted than 





thought a few 



generations ago. 



Whether it car 



1 be reduced to 



zero may per- 



