IDEAS OF SOLIDITY. 397 



ward, but not forward, and hence there arises a correction for Lateral inver , 

 the lateral inversion. This, of course, implies the existence sion corrected 

 of some structural arrangement which shall either correspond- y 

 ingly correct the vertical inversion, or bring back the lateral to its orig- 

 inal erroneous state, and thereby establish a harmony of position in the 

 two directions ; and if, in the retina itself, the means exist for the cor- 

 rection of inversion, vertical as well as lateral, by changing the direction 

 of the conducting tubules, it necessarily must be that that place of cor- 

 rection is where the retina is intersected by the optical axis of the eye. 

 I think it is to be greatly regretted that we are not bet- Suggestion re- 

 ter acquainted with the construction of the yellow spot of JSJjJj^Jfjl 

 Soemmering, which occurs at this very point. The ridge- Soemmering. 

 like form it presents, the thin, uncolored spot in its centre, its more def- 

 inite occurrence in those animals, as man, the quadrumana, and some 

 saurians, the axes of whose eyes are nearly parallel to one another, seem 

 to indicate, in a very significant manner, that at this place the correction 

 in question is made. There are many ways in which we may conceive 

 this to be done by varying the direction of the nervous tubules. As an 

 illustration, it may be remarked that if, through a small hole made in a 

 sheet of paper, a number of threads, the end of each of which is fasten- 

 ed to the back of the sheet, be caused to pass, under the condition that 

 they do not cross one another in the hole, but leave its aperture open, 

 their direction in space as they retire from the hole will be inverted as 

 respects the direction in which they approached to it. The analogy be- 

 tween such an aperture and the foramen of Soemmering is too striking to 

 be overlooked. 



The stereoscope, invented by Professor Wheatstone, shows to what an 

 extent our ideas of the solidity of objects depend on the dif- 

 ferences of the images in each eye. By reason of their dif- 

 ference of position, each of the two eyes will have a different picture upon 

 its black pigment of any solid object, and the mind, combining these dis- 

 similar pictures into one, gathers therefrom the idea of solidity. If thus 

 we offer to the eyes two pictures of a given object, presenting the same 

 form as that object would have done when seen from each eye respect- 

 ively, the mind combines these flat pictures together, and can not divest 

 itself of the idea of a solid body. This is the principle of the stereo- 

 scope. It is shown by this instrument that, when two such pictures of 

 different sizes are used, the mind combines them into one of intermediate 

 magnitude. Probably this effect is involved in the circumstance that, 

 when we look at an object unequally distant from the two eyes, we still 

 see it single. When two images of different colors are employed, the 

 mind can not combine them, but sees first the one and then the other, 

 the brightest one continuing the longest. 



