84 Intelligence and Miscellaneous Articles. 



we can obtain either hollow or solid pyramids anywhere between 

 the two perspectives ; or by a finger on both eyes, or nnder both, 

 we can obtain the pyramid and its " converse " below or above both 

 perspective outlines. 



In all these cases the optic axes do not intercept each other, but 

 the axes of visible direction (functional of the final directions of 

 the optic nerves) do meet on the paper ; that is, the first pair of 

 axes are not, and the second pair are, in such a realization as is herein 

 planned, in one plane. 



In all these cases the perspectives fall on similar parts of the 

 two retinse, as in the modes originally mentioned by Wheatstone. 



The author goes on to consider in what way the sensorium refers 

 the sensations it receives notice of from the optic nerves into space, 

 so as to fix the place, size, and form of an object. 



The theory of vision in retinal normals being proved to be un- 

 tenable, it is admitted that there is some such association of the 

 two retinse as to have fairly suggested the theory of "identical" or 

 "covering" points; but this relation he believes to be subordinate 

 to a law by which the sensorium projects or emits its perceptions into 

 space, as it were in two imaginary cones of sight-rays, which, though 

 not issuing from the ends of the optic nerves, have apices whose 

 positions are functional of the directions of these ends for the instant 

 in question — and that it is by the intersection of the sight-rays in 

 these cones, limited by the law of similar retinal parts, that the places, 

 sizes, and forms of objects are determined. Hence, if we conceive 

 that a pair of stereoscopic perspectives, one being imaged on one 

 retina and one on the other, exist as sight-affections in miniature 

 in the substance of the optic nerves, the size of the resultant solid 

 form will be greater the greater is the distance from the nerves at 

 which the axes of visible direction intersect, or the optic axes when 

 they are coincident respectively with them. 



The paper concludes by exemplifying in sundry ways the modes 

 in which the conclusions in it may be applied in investigating seem- 

 ingly anomalous phenomena in physiological optics. 



IX. Intelligence and Miscellaneous Articles. 



INDUCED CURRENTS AND DERIVED CIRCUITS. 

 BY JOHN TROWBRIDGE. 



THE expression for the intensity of an induced current, deduced 

 1 dll 

 by Neumann and Sir William Thomson, is as follows : — fc—j: -jt-> 



in which Jc is a coefficient depending upon the resistance of the 

 complete wire in the secondary circuit, and U is a certain " force- 

 function " which depends solely upon the form and position of the 

 wire at any instant, and on the magnetism of the influencing body. 

 The expression, in general language, is as follows : — 



" When a current is induced in a closed wire by a magnet in 

 relative motion, the intensity of the current produced is proportional 



