334 



SCIENCE. 



[Vol. XXI. No 541 



lower levels until it falls to its real place on the floor. The com- 

 bination beyond the plane of the object, and therefore with fig- 

 ures enlarged, is also easy if the figures are small, but never 

 quite so easy as combination on the nearer side. 



These phenomena are as easy to me as any ordinary act of 

 sight. No device of any kind, such as use of pencil or finger to 

 fix the point of sight is at all necessary. I can watch the double 

 images approach, combine, pass over, combine with the next 

 figure, etc., with the greatest ease and certainty. Moreover, the 

 sense of reality and of exact distance is as perfect as that of any 

 other object. 



In young normal eyes great difl3culty is often experienced in 

 getting this perfect perception of distance because the phantom 

 is not perfectly clear. The reason is this: The two adjustments 

 of the eyes, the axial and the focal, are invariably associated in 

 every act of sight. Therefore, in the experiment the eyes are ac- 

 commodated to the point of ocular convergence, i.e., thedistanceof 

 the phantom. But the light comes from a greater distance, viz., 

 from a real object — the floor. The retinal image, therefore, is not 

 distinct and the figures are blurred. I no longer, now, suffer 

 from this difficulty, because I have become presbyopic, and have, 

 therefore, lost the power of accommodation. The clearness of 

 the phantom is perfect almost immediately. When I was younger, 

 there was always a considerable interval before the phantom be- 

 came clear. The clearing up was the result of a dissociation of 

 these two consensual adjustments. While the axial adjustment 

 remained adapted for the distance of the phantom, the focal ad- 

 justment (accommodation) was changed to the distance of the 

 real object. Now this dissociation of two adjustments so invaria- 

 bly associated in every act of sight, is diflficult for most, and im- 

 possible for many persons. But until this dissociation is effected 

 and the phanton becomes perfectly clear, the sense of reality, and 

 especially the perception of distance, will be imperfect and 

 vacillating. The use of glasses adapted to distinct vision at the 

 distance of some one of the possible phantoms will make that 

 particular phantom clear. 



Notv this clear perception of the distance of a phantom, nearer 

 and SQialler iu proportion to the degree of ocular convergence, is, 

 of course, not peculiar to me. All writers on the subject record 

 the same result. All my pupils who succeed at all in binocular 

 combinations get the same result. I am sure I have tried hun- 

 dreds, I might almost say thousands, and always with the same 

 result. This result is, therefore, normal and in complete accord 

 with the laws of vision. For near objects, there are two modes 

 of estimating distance, viz., by axial convergence (binocular per- 

 spective) and by accommodation (focal perspective). Now, of 

 these two, the former is by far the more exact, and therefore 

 takes control of judgment of distance if the two are not in accord. 

 This is proved by naked-eye combination of ordinary stereoscopic 

 pictures by ocular convergence. In such cases, we have the phe- 

 nomenon of inverse perspective. The judgment of relative dis- 

 tance by axial convergence completely reverses the real relative 

 position of objects. Binocular perspective overrides every other 

 form of perspective, whether focal, or mathematical, or aerial, 

 and comes out victorious in spite of the absurdity or even impos- 

 sibility of its results.' 



Now, in the case of phantoms, axial convergence fixes the dis- 

 tance. But this fixes also the size; for the apparent size of any- 

 thing is a product of the retinal image multiplied by the estimated 

 distance. The size of the figures will be small in proportion to 

 the nearness of the phantom. This is in exact accord with the 

 laws of vision. But Mr. Bostwick says, that in his case the fig- 

 ures seem smaller and yet more distant than the real object. He 

 explains this, if I understand him aright, by the fact that in the 

 dissociation of the axial and focal adjustments, while most per- 

 sons follow the axial, he follows the focal adjustment, in esti- 

 mating distance. Near objects require greater accommodation; 

 but there is no such accommodation in this case, therefore the ob- 

 jects judged by this test will not seem nearer. But, again, since 



' If anyone is specially Interested in this subject, he will find It fully 

 treated In my little volume, entitled "Sight," volume 31 of the International 

 Scientific Series. 



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