Elementary Relations between 'Electrical Measurements. 507 



The two great facts of hemiopsy in both eyes, and of what is 

 called single vision with two eyes, do not require the hypothesis 

 of semidecussation to explain them. If hemiopsy is produced 

 by the distended blood-vessels of the retina, these vessels must be 

 similarly distributed in each eye, and similarly affected by any 

 change in the system ; and consequently must produce the same 

 effect upon each retina, and upon the same part of it. 



In explaining single vision with two eyes, we have no occasion 

 to appeal to double fibres in the optic nerves, or to corresponding 

 points on the retina. There is, in reality, no such thing as single 

 vision, that is, a single image seen by both eyes. With two 

 sound eyes every object is seen double, and it appears single only 

 when, by the law of visible position, the one image is placed 

 above the other. But even in this case the object is seen double, 

 by means of two dissimilar images of it which are not coincident. 

 By shutting the right eye we lose sight of a part on the right 

 side of the double image, which is seen only by the right eye.; 

 and by shutting the left eye we lose sight of a part on the left 

 side of the double image, which is seen only by the left eye. If 

 one eye gives a better picture than the other, the duplicity of the 

 apparently single image is more easily seen. By shutting the 

 good eye the imperfect picture is seen, and by shutting the bad 

 eye we insulate the perfect picture. It is difficult to understand 

 how optical writers and physiologists should have so long de- 

 manded a single sensation for the production of a single picture 

 from the two pictures imprinted on the two retinas. If we had 

 the hundred eyes of Argus, the production of an apparently 

 single picture would have been the necessary result of the Law 

 of Visible Position. 



LXX. On the Elementary Relations between Electrical Measure- 

 ments. By Professor J. Clerk Maxwell and Fleeming 

 Jenkin, Esq. 



[Concluded from p. 460.] 



Part IV. — Measurement of Electric Phenomena by Statical 

 Effects. 



33. f]LECTRO STATIC Measure of Electric Quantity.— -By 

 the application of a sufficient electromotive force between 

 two parts of a conductor which does not form a circuit, it is possi- 

 ble to communicate to either part a charge of electricity which may 

 be maintained in both parts, if properly insulated (14). With 

 the ordinary electromotive forces due to induction or chemical 

 action, and the ordinary size of insulated conductors, the charge of 



2L2 



