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SCIENCE 



[N. S. Vol. XXXIV. No. 887 



binokenlaren Instrumente." Both text and 

 bibliography are concerned predominantly 

 and designedly with the physical problems of 

 stereoscopy, though the discussion of the re- 

 sulting refinements and variations of the 

 stereoscopic effect reflects indirectly upon the 

 psychological problems. While involving at 

 each step questions of psychological analysis 

 and theory, the essential advances in stereo- 

 scopy have been physical in nature. In part 

 they constitute the physical solutions of prob- 

 lems raised by the study of depth-perception; 

 and in yet larger measure they constitute 

 original physical problems of application, ex- 

 tension, and quantitative refinement of the 

 stereoscopic principle. However, the existence 

 of an adequate psychological study contributes 

 to the physical problems a very different 

 status than attached to them fifty years ago, 

 when it was difficult to convince the scientific 

 public that psychology had any logical right 

 or proprietary interest in an instrument made 

 of prisms, or lenses, or mirrors. 



The renascence of interest in stereoscopic 

 problems is abundantly evidenced by the ex- 

 tent of the literature, and further by the great 

 variety of publications in physiological, 

 psychological, ophthalmological and general 

 scientific journals, together with applications 

 of stereoscopic presentations to scientific, edu- 

 cational and technical procedure. While the 

 expository article of Dr. Pullfrich touches 

 upon but few of these phases of the subject, it 

 is written with a background reference to 

 them, as a support of the interest in the prob- 

 lems considered. It seems the irony of fate 

 that the man who by construction and analy- 

 sis had done so much to make possible the re- 

 finements of stereoscopic vision, is himself 

 deprived of their enjoyment. Having lost the 

 use of one eye. Dr. Pullfrich records that to 

 him the beauties of stereoscopic effects are a 

 matter of remembrance only. 



Pullfrich's exposition is itself so condensed 

 that this notice may be confined to an ac- 

 count of its method and procedure. The 

 fundamental condition of stereoscopic vision 

 is the separation and position of the eyes in 

 the head, the variations of which in different 



animals offer suggestive and as yet incom- 

 pletely interpreted potentialities of depth- 

 perception. The part of the visual field in 

 which stereoscopic vision operates is limited, 

 and makes the reports of space-relations from 

 those portions of the visible world which the 

 right eye and the left eye respectively but 

 exclusively survey, a special problem of in- 

 direct stereoscopy. Obviously such report is 

 momentary and shifting, since a turn of the 

 eyes brings the outlying object into the binoc- 

 ularly policed territory. As to the physiolog- 

 ical or psychological basis of the team-work 

 which the two eyes so marvelously perform, we 

 are reduced to ingenious hypotheses. The 

 principles of stereoscopic vision express merely 

 the conditions of conformity necessary to the 

 production of the depth-effect, and the corol- 

 laries of variation in effect resulting from 

 shifting values of the many variable contribu- 

 tors. It has become clear that the presumable 

 alternative of the earlier discussions between 

 the part played by " retinal dissimilarity " 

 and by " convergence shif tings," is not an ex- 

 clusive one; the two jointly contribute to the 

 effect in practise, and this circumstance re- 

 flects back upon the theory, by suggesting 

 that tentative motor initiatives may even fuse 

 with seemingly instantaneous retinal im- 

 pressions. 



The physical problems are, in a sense, con- 

 ditioned by the marvelous precision of the 

 psychological depth-perceiving mechanism ; 

 for were not the optical instrument sup- 

 ported by the visual fineness of distinction, 

 there would be no possibility of the utiliza- 

 tion by the eyes of the extensions and con- 

 trols of its verdicts which the inventions of 

 Dr. Pullfrich and the constructions of the 

 firm of Zeiss have added to the triumphs of 

 science. The problems arising from the re- 

 construction of a natural depth-effect from 

 the combination of two photographic (or 

 diagrammatic) views — the divergence of 

 which reproduces the difference resulting 

 from the base line of the interocular distance 

 — are naturally distinct from those growing 

 out of the project of extending the range or 

 degree of depth-perception of an actual and 



