188 Prof. J. LeConte on some Phenomena 



some distance from the observer, the axes of the eyes may be re- 

 garded as nearly parallel as II c } I b. I now by a voluntary effort 

 bring the point of sight along the line II c nearer and nearer, 

 until it reaches a near the root of the nose. In doing so the 



spectrum is always seen to incline to the left, thus\ . On re- 

 laxing the convergence and looking again at the wall, the spec- 

 trum retains its inclined position for an appreciable time and 

 then gradually recovers its original verticality. In similar ex- 

 periments with the right eye the spectrum is always seen to in- 

 cline to the right, thus / . 



I next tried direct observation of the eye itself. As I could 

 not find ony one with the necessary control over the eyes, I 

 was compelled to make myself the subject of this observation. 

 While, therefore, with the right eye shut I gaze with the left eye 

 across the root of the nose on vacancy, or on a distant object as 

 in the figure (fig. 9), an observer, conveniently placed near the 

 visual line, carefully examines the iris of my eye so as to recog- 

 nize the position of the radiating lines. When now, without 

 changing the position of the visual line of the left eye, I turn the 

 right eye inward as in the previous experiment, until the point 

 of sight is at a, the globe of the left eye is distinctly seen to ro- 

 tate outward. I got four different persons to make this obser- 

 vation upon my eye, and the testimony of all was the same. 



I had proceeded thus far in my experiments when I was led 

 to reflect further upon the phenomena presented by the diagram 

 in which the lines were highly inclined. In this diagram, it will 

 be remembered, the verticals were combined with more facility 

 than the horizontals. I now repeated all my experiments with 

 more care and with especial reference to this point. As I ex- 

 pected, I found the same true for all the diagrams ; but the dif- 

 ference was so small that it had escaped detection. This led me 

 to suspect that there might be some truth in Professor Helm- 

 holtz's principle of real and apparent vertical. I therefore con- 

 structed many other diagrams to test this point. I constructed 

 first a diagram exactly like fig. 6, except that the circles were the 

 same distance apart as my eyes, viz. 2\ inches. On placing this 

 diagram before me and gazing on vacancy, the eyes therefore in 

 their primary position, the circles were brought together. In 

 this experiment the verticals came together ■parallel. I sometimes 

 thought there was a scarcely perceptible inclination in the direc- 

 tion required by Helmholtz's principle, viz. thus / \ . If any 

 such inclination really existed, it could not have been more than 



