On the Phantascope. 155 
double AA AA ; continuing to raise the screen and to regard 
the index, the double images will recede more and more until 
their position will be thus A Act A A; continuing 
still to raise the screen, the two internal images approach until 
they are optically superimposed and coalesce into one, thus; 
A A 
This middle or superimposed figure, is the phantom or image 
where there is really no object. Cease to look at the index I, 
and turn the attention to the base-board itself, and this phantom 
figure instantly vanishes. If the two letters be placed on the base- 
oard at the same distance as the eyes are apart, say two and a 
half inches, then this normal position of the screen will be just 
all way between the eyes and the base-board. If they are pla- 
ced further apart, the screen must be raised higher; the distance 
from the eyes to the index screen being in all cases, to the dis- 
tance from that screen to the base-board, as the distance between 
the eyes is to the distance between the objects viewed. In the 
case above, the phantom image is formed exactly as if there were 
@ letter in the area of the index screen of half the size of the prir- 
itive letters on the base-board, and optically the letter should ap- 
pear then, but the knowledge of the observer that there is nothing 
at that place, will often prevent the deception. 
. 2.—Lay upon the base board a card having letters or 
other figures which are identical in size and form set in regular 
Tows and at equal distances all over, thus: 
Avsvter Ae Biody od 
te. Gee ae as wae 
AO] a BO! 
and proceed to raise the screen as before; you will form phantom 
mages as before between each of these figures, or possibly you 
Will superimpose the first object upon the third, when you will 
nave, not a single phantom but a whole plane of them, each pair 
resenting a phantom between. This phantom surface will be 
ikely to eflect a complete deception, and will rise from the base- 
board and coincide with the index plane, when it may be con- 
templated 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. 
