Prof. Locke on the Phantascope. '^SB 



being in all cases, to the distance from that screen to the base- 

 board, as the distance between the eyes is to the distance be- 

 tween the objects viewed. In the case above, the phantom 

 image is formed exactly as if there were a letter in the area 

 of the index-screen of half the size of the primitive letters on 

 the base-board, and optically the letter should appear then ; 

 but the knowledge of the observer that there is nothing at that 

 place will often prevent the deception. 



Exp. 2. Lay upon the base-board a card having letters or 

 other figures which are identical in size and form, set in re- 

 gular rows and at equal distances all over, thus : 



A A A A A A 

 A A A A A A 



A A A A A A 



and proceed to raise the screen as before ; you will form phan- 

 tom images as before between each of these figures, or possibly 

 you will superimpose the first object upon the third, when you 

 will have, not a single phantom, but a whole plane of them, 

 each pair presenting a phantom between. This phantom 

 surface will be likely to effect a complete deception, and will 

 rise from the base-board and coincide with the index plane, 

 when it may be contemplated with the same deliberation and 

 ease to the eyes as if it were a real object. This would be 

 sure to be the case if the index plane were figured over in the 

 same manner, but with figures properly reduced in size. 



Exp. 3. Place two identical pictures of the same flower on 

 the base-board, say they are an inch in diameter, and two 

 and a half inches apart; place also on the edge of the index 

 area a picture of a small flower-pot or vase, with flower stems 

 as an index ; then form the phantom image as before, and the 

 flowers will appear in the vase so long as you contemplate the 

 stems at the index-screen ; but the moment the eyes are di- 

 rected to the flowers themselves, the phantom vanishes. 



Exp. 4-. Let one of the above flowers be red and the other 

 blue ; the phantom will be purple. Sometimes, however, it 

 will appear nearly red, and then again blue. This and some 

 other experiments convince me that the attention of the mind, 

 even when we are looking with both eyes, is often directed 

 exclusively to the image in one eye ; and perhaps after that 

 tires, the image in the other is contemplated. Jf this be true, 

 then the two eyes serve in the first place to fix the distance 

 of an object by the amount of convergence, and in the next 

 place to relieve each other by turns. 



