186 



Prof. J. LeConte on some Phenomena 



horizontals were inclined so as to make an angle of 1 \ Q with 

 the true vertical and the true horizontal (fig. 8), and therefore 

 perfect squares with one another. At the proper distance, when 

 the small circles were brought together, the coincidence of both ver- 

 ticals and horizontals seemed to be perfect. When the plane of the 

 diagram was too near or too far, all the lines crossed, in the one 

 case in one direction and in theother case in the other direction. 

 I then constructed still other diagrams, in which the inclination 

 of the lines with the true vertical and the true horizontal were 

 40 minutes, 2^ degrees, and 5 degrees. In all cases I brought 

 the lines into coincidence, but of course by different degrees of con- 

 vergence. In the last the optic convergence necessary was ex- 

 treme, and the strain on the eyes considerable ; but in the other 

 cases there was not the slightest difficulty or strain. Recollect- 

 ing, however, that Professor Helmholtz supposed that the change 

 of position of the horizontals might be the result of fatigue, I tried 

 repeatedly after long rest, but always the phenomena were pre- 

 cisely the same. In the diagram in which the inclination of the 

 lines was 5 degrees I observed, however, that a greater degree of 































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convergence was necessary to bring the horizontals into coincidence 

 than to bring the verticals into coincidence. The difference in the 

 distance of the diagram in the two cases was about two inches, 

 and the difference in the distance of the point of sight was about 

 half an inch. I cannot explain this except by supposing that 

 the form of the optic globe was changed by the excessive action 

 of the muscles. 



I can conceive of no possible source of fallacy in these experi- 

 ments. From long practice they have become almost as easy to 

 me as any ordinary act of vision. They do not now fatigue my 

 eyes in the slightest degre 



I see the lines of the two images. 



