NATUKAL PHILOSOPHY. 149 



ments described by the Professor, the lines should be very strongly illuminated, 

 and the observer snould have some practice in experiments on subjective vision. 

 But the following is a more simple proof that pictures successively impressed on 

 the two eyes are sufficient for the stereoscopic effect : — Let a screen be made to 

 vibrate or revolve somewhat rapidly between the eyes and the twin pictures of a 

 stereoscope, so as alternately to expose and coxer each, completely excluding the 

 simultaneous vision of the two. The stereoscopic relief will be as apparent in 

 these conditions as when the moving screen i3 withdrawn. Here there is no 

 opportunity for the combination of pairs of corresponding points in the two dia- 

 gramsbythe simultaneous convergence of theoptic axes throughthem: but at each 

 moment the actual picture in the one eye and the retained impression in the other, 

 form the elements of the perceptive resultant perceived. In repeating, with suc- 

 cess, the curious experiments of Professor Dove, to obtain the stereoscopic effect 

 by the momentary illumination of the electric flash, Professor Rogers found great 

 advantage in using one of Ritchie's improved Ruhmkorff's coils, having a coated 

 jar included in the outer circuit, the intensely brilliant spark of which can be 

 made to throw its light upon the object viewed, in any direction or at any inter- 

 vals that may be desired. 



From the facts that the duration of an electric spark is less than one-millionth 

 of a second (Wheatstone), and that we are able by a single flash of lightning to 

 perceive the solidity and relief of an object to which the eyes are directed, — we 

 may conclude that the perception of an object in its proper relief doe3 not neces- 

 sarily require the eyes to be converged upon every visible point of it in succession, 

 and that the perception of the perceptive resultant, through binocular combina- 

 tion in a stereoscope or otherwise, may arise directly from the two pictures 

 impressed, without the necessity of combining, pair by pair, all the corresponding 

 points of the component lines or drawings, jsor is it necessary that the images 

 of the corresponding points of the objects should fall on what are called corre- 

 sponding points of the retina. The condition of single vision in this case seems to 

 be simply this, that the picture in the two eyes shall be such and so placed as to 

 be identical with the picture, which the real object would form, if placed at a 

 given distance, and in a given attitude before the eyes. 



Prof. Rogers has shown that the law of binocular vision is valuable 

 in examining bank-notes. Put a genuine note in one compartment 

 of a stereoscope, and a counterfeit note in the other, and every 

 difference will be readily distinguished. 



THE HUMAN EAF,. 



Mn. Toynbee, F.R.S., has communicated to the Royal Society a 

 paper "On the Mode in which Sonorous Undulations are conducted 

 from the Membrana Tympani to the Labyrinth, in the Human Ear." 



The opinion usually entertained by physiologists is that two chan- 

 nels are requisite for the transmission of sonorous undulations from 

 the membrana tympani to the labyrinth, viz., the air in the tympanic 

 cavity which transmits the undulations to the membrane of the 

 fenestra rotunda and the cochlea ; and secondly, the chain of ossicles 

 which conduct them to the vestibule. 



This opinion is, however, far from being universally received : 

 thus, one writer on the physiology of hearing contends that "the 

 integrity of one fenestra may suffice for the exercise of hearing ;" 

 another expresses his conviction that "the transmission of sound 

 cannot take place through the ossicula ;" while Sir John Herschel, in 

 speaking of the ossicles, says, " they are so far from being essential 

 to hearing, that when the tympanum is destroyed and the chain in 

 consequence hangs loose, deafness does not follow." 



The experiments and observations detailed in Mr. Toynbee's paper 

 lead to the following conclusions : — 



