200 



Prof. J. LeConte on some Phenomena 



diagram at the distance of seven inches from Fig- 14 



the eyes, the two lines are brought into per- m 

 feet coincidence. If, on the contrary, the 

 same figure be turned upside down and the H 

 eyes be placed a little further than seven 

 inches, so that the two points a, a are brought 

 together by crossing the optic axes at the 

 distance of seven inches, then also the lines 

 are brought into perfect coincidence. The 

 accompanying figure (fig. 15), in which 0, 0' 

 are the eyes, A the point of sight, a H, a H, 

 and a! H', a! H' are the lines in the two po- 

 sitions, will explain how the stereoscopic 

 combination takes place in each case. The 

 line H'AH is the horopter. This experi- 

 ment is difficult to perform satisfactorily. 

 When the lines come together it is difficult 

 to determine whether there is real coinci- 

 dence or not. I have observed, however, 

 that when the coincidence is not perfect the B 

 white and black lines seem to run spirally 

 round each other. The best plan is to observe them at the mo- 

 ment of coming together or of separating. I feel quite confident 

 of the reliability of the conclusions reached. 



Fig. 15. 



I made many calculations, based upon these experiments and 

 on the previous experiments on the rotation of the eye, to deter- 

 mine the inclination of the horopteric line for different degrees 

 of convergence, i. e. for different distances of the point of sight. 

 The results of these calculations were not entirely satisfactory. 

 I had expected from Meissner's results that there would be found 

 a progressive increase as the distance decreased. But I could 

 not be sure from my calculations of any increase or decrease 

 with distance. For all distances the inclination seemed to come 



