332 Archdeacon Pratt on a Problem in 



allow that the impression even with blended double images* is 

 different, according as we have to do with a crossed or an un- 

 crossed one, with a greater or a less distance, experience is 

 quite sufficient to explain stereoscopic vision. For as points 

 which lie on the other side of the fixed point always produce 

 homonymous, those on this side crossed double images, experi- 

 ence must gradually lead us to combine with the impression of 

 an homonymous blended double image the conception of a more 

 distant, with that of a crossed one that of a nearer object. The 

 distance of the double image will give in this way a measure 

 for the greater or less distance of the object. 



I imagine that in this I have explained essentially the pro- 

 duction of the perception of solidity ; that other circumstances 

 (such, for instance, as aerial perspective, or the position of the 

 optical axes) have at any rate a subordinate influence is well 

 known. Hence I imagine that the following results have been 

 arrived at by these investigations : — 



1. Under ordinary circumstances the images of most objects 

 which are simultaneously visible are projected upon almost 

 identical places. 



2. This continuous simultaneous use for the same purpose 

 justifies us in considering that the peculiar deportment of these 

 places is an acquired one. This view is moreover supported by 

 various other reasons. 



3. Considered from this point of view, the explanation of 

 binocular perception of depth presents no difficulty, if it be 

 allowed that it depends upon the impression which two blended 

 images produce, according as they are crossed or homonymous, 

 and more or less distant. 



XLV. To find what changes may be made in the arrangement of 

 the mass of a Body, without altering its outward form, so as not 

 to affect the attraction of the whole upon an external point. By 

 Archdeacon Pratt, M.A., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



ALTHOUGH I have already sent you two papers on this 

 topic, I do not scruple to ask you to publish a third, 

 because the subject is one of great importance in the fluid-theory 

 of the earth's figure. 



1. Suppose an Imaginary Body formed by subtracting the 

 given body in its original state from the same body with its ma- 



* I may he permitted to speak of a double image whenever two points 

 of the same object occur on the retina, without reference to the question 

 whether in fact actually separate images come to the consciousness or hot. 



