AMERICA! JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



In previous papers the phenomena of optic <Ii\ ergenee have 

 been discussed, and also various peculiarities of vision under 

 controllable physiological conditions. Among them was ste- 

 i't;<»scopy from a pair of perfectly similar figures, produced by 

 so varying these in relative position that the retinal images of 

 them were dissimilar. A geometric explanation of this was 

 given, in which it was assumed that freedom of motion was 

 allowed the eyes ; but with the reservation that such motion is 

 not necessary in obtaining the perception of binocular relief 

 from stereographs constructed in the ordinary way. and that it 

 was probably necessary only to the completeness of the percep- 

 tion in the present case 

 was very considerable. 



In continuing this investigation the electric spark has been 

 employed as a means of illuminating the pair of pictures. 

 These were viewed with the aid of a reflecting stereoscope, 

 already described as a device to indicate the value of the optic 

 angle, positive or negative, that results from any possible rela- 

 tion between the visual lines of a pair of eyes. Vision may 

 thus be made normal or abnormal at will. The use of the 

 spark in the study of binocular vision is no novelty; but it 

 seems not to have been employed hitherto in studying abnor- 

 mal vision with the visual lines divergent, or the peculiar mode 

 of stereoscopy recently devised. 



The apparatus employed was a large induction coil, belong- 



