454 Prof. Locke on the Phantascope. 



having neither lenses, prisms nor reflectors, the object being 

 in general the same as holding the finger or other object near 

 the eyes and concentrating the attention upon it for the pur* 

 pose of optical convergence. 



It consists of a flat base-board about nine by eleven inches, 

 with an upright rod at one end bearing two sliding sockets to . 

 be clamped at any elevation like those of a retort stand, or 

 adjustible by stiff' sliding springs. The upper socket supports 

 horizontally a small vane or card, having a slit or sight-hole 

 one-fourth of an inch wide and three inches long from right 

 to left. This slit has its middle directly over the centre of 

 the base-board, and is intended to have the eyes directly over 

 it, one eye at one end and the other at the other ; two small 

 holes, say one-fourth of an inch, occupying the place of the 

 ends of this slit would answer, except for the unequal distances 

 between the eyes of different observers. The lower of the 

 two sockets bears horizontally a moveable screen of paste- 

 board or thin wood, having a slit at least three inches wide 

 from left to right, and about one inch in the other direction, 

 with its centre also perpendicular over the centre of the base- 

 board. This screen has marked vertically across its middle 

 an index, shown by two arrows in the figures marked I. 



Experime7its. — In experimenting with the phantascope, the 

 operator places whatever is to be tried upon the lower tabular 

 base-board, looks downward through the upper slit and slides 

 the screen up or down until he attains the adjustment required. 



Exp. 1. Let there be two identical letters, say A, placed 

 or drawn on the base-board about two and a half inches apart 

 from left to right, let the moveable screen be nearly down, 

 and directing the eye, not to the letters, but to the index, draw 

 the sliding-screen bearing the index gradually upward ; the 

 two letters seen indirectly will appear as four, or each letter 

 will bo 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 A 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 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-board 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 half-way between the eyes and the base- 

 board. If they are placed further apart, the screen must be 

 raised higher; the distance from the eyes to the index-screen 



