1868. 95 



[Jeffries. 



ing at once single and distinct vision of every object, are two actions 

 necessarily simultaneous and inseparably connected. They are there- 

 fore both, each in its way, criteria of the distances of objects; but 

 they give rise to certain indirect and additional criteria for other 

 distances, in two ways: one, the more important, is the double miao-es 

 of the objects situated before and behind the point of convergence; 

 and the other, but only in a subsidiary way, the degree of confusion 

 of the objects situated before and behind the point of convergence, 

 and which are not in focus." This latter, as he says, "being monocu- 

 lar also, should perhaps be lei't out in considering binocular vision. 

 Therefore it is particularly the sensation of the double images, the 

 degree of their separation, and their respective po; itions, either out- 

 side or inside from the centres of the two retinte, which indicate more 

 powerfully the exact distance of the object from the point of single 

 vision, either before or behind. When we look fixedly on a point of 

 one surface of the revolving card, that point appears single, and we 

 see at the same time another point on the other surface which appears 

 double, although we hardly feel that we notice its doubleness ; and 

 from the j)osition or distribution of the double images, either on the 

 right or on the left of the central jjoint, we have at the same glance 

 the perception of the respective distances. Therefore to judge of 

 the distances of certain objects in the direction of the line of vision, 

 we are not absolutely obliged to alter constantly the angle of con- 

 vergence. This is proved by our perception of the two distances of 

 the surfaces of the card wljile it is revolving; for it would be impossi- 

 ble that we should alter the angle of convergence to adapt it alter- 

 nately to the two surfaces while they are turning so rapidly. The 

 same angle of convergence kept on one or the other surface is no im- 

 pediment to our seeing both in a sufficiently distinct manner." My 

 diagram illustrates this as Mr. Claudet puts it. 



However, 1 get this effect or diflference of plane Avith one eye, 

 heightened by binocular vision. To me it seems, therefore, that it 

 must depend on accomodation rather than on change of convergence. 

 I admit the intimate relation of these two muscular efforts, though 

 the researches of Prof. Bonders show that they can be dissociated 

 more than we should gather from Mr. Claudet's remarks. I am aware 

 how much the muscular effort for convergence tells us in regard to 

 distance, to prove which I know of no more striking experiment than 

 that of Meyer's (Vide Archiv fur Opth. Bd. 2, 1856), Avith the 

 series of strings in different planes, as I have shown to the Society. 



