106 Mr. J. Elliot on two new Forms of the Stereoscope, 



slips of wood sliding vertically, so as to show nothing more than 

 the breadth of the pictures. Some minute object is then attached 

 to the margin of the aperture towards which both eyes are directed, 

 and the two pictures are then seen in stereoscopic relief, stand- 

 ing out as a miniature landscape in the air between the pictures 

 and the eye. The effect is exceedingly beautiful, but it is not 

 every person that can succeed in seeing it. When the eyes alone 

 are used for uniting pictures, three pictures make their appear- 

 ance, the middle one, in which two are united, giving the ste- 

 reoscopic effect. The object of the instrument in this form, is to 

 hide the two exterior pictures, or rather not to permit them to 

 be formed. 



Since the left-hand picture is hidden by the instrument from 

 the left eye, and the right-hand picture from the right eye, each 

 eye sees only the picture on the other side ; the axes of vision 

 evidently cross each other, and the various points of the land- 

 scape appear where the visual rays from the same points on the 

 two pictures intersect each other, and consequently between the 

 pictures and the eyes. 



In uniting the pictures in this way, a curious physiological 

 phsenomenon presents itself. When the pictures first unite, the 

 eye is tolerably well satisfied with the result, but not thoroughly 

 so, and the two pictures retain a considerable disposition to part 

 company. But in a second or two a peculiar change is felt in 

 the eye ; the landscape starts into sudden clearness, and the two 

 pictures seem afterwards ^cTcc? together, having no tendency what- 

 ever to separate. The cause, I believe, is this : — At the first union 

 each eye spontaneously adapts its focal adjustment, as it has 

 always been accustomed to do under natural circumstances, to 

 the distance at which the directions of the axes intersect each 

 other. In that state, of course, union of the pictures may be 

 effected, but accompanied with obscurity, for any object situated 

 at a different distance from that to which the eye has adjusted 

 its focus will be seen indistinct. The eye being uneasy under 

 the defect of which it is sensible, changes its configuration, and 

 soon finds ou.t the best attitude to assume, viz. with the axes of 

 the eyes converging towards one distance, and the focal adjust- 

 ment adapted to another, — a combination which never takes place 

 under natural circumstances. 



The landscape formed in this way being so minute, upon Mr. 

 Waterston's inquiring if it could not be magnified, I immediately 

 proposed the inti'oduction of small telescopes ; and, in fact, I 

 had previously had the slides with the wide apertures made on 

 purpose to permit the introduction of some magnifying power, 

 which I soon saw could only be telescopic, from the distance at 

 which the pictures were to be viewed. We ultimately, however. 



