HERMANN VON HELMHOLTZ 



numerous ingenious experiments, and containing a 

 mathematical treatment of the subject. In 1864, he 

 delivered the Croonian Lecture to the Royal Society 

 a famous lecture 'On the Normal Motions of 

 the Human Eye in relation to Binocular Vision,' and 

 this was followed in the same year by two papers 

 on the horopter. In the following year, 1865, he 

 published a paper on the Influence of the Orienta- 

 tion of the Eyeballs on the Projection of the Retinal 

 Pictures, another on Stereoscopic Vision, and a third 

 on the Movements of the Eyeballs. At the Congres 

 periodique internationale d'Ophtalmologie a Paris, in 

 1867, he read a paper on Stereoscopic Vision and 

 the Sense of Relief. He returned to the question in 

 1878, after he became Professor of Natural Philosophy 

 in Berlin, and even so late as 1881, in his sixtieth 

 year, he published a note in the Philosophical Magazine 

 on the same subject. 



It is somewhat difficult to give an account of the 

 results of those researches in untechnical language, 

 but it is only fair to Helmholtz to make the attempt. 

 A picture of an external object is formed on the 

 retina of each eye by the lens-like structures placed 

 in front of it, in accordance with the laws of dioptrics. 

 The two pictures, however, give the sensation of one 

 object. Further, we can move the eyeballs so as 

 to direct them to the object we wish to examine. 

 Thus we can move them simultaneously up to the 

 heavens or down to the earth, or to the right side 

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