WHEATSTONE AND BREWSTER. 



imagined to be continued backwards, they will meet at a certain point 

 behind the reflectors ; and if the drawings r and I be made to correspond 

 with the views which the right and left eyes would have respectively of the 

 object itself, which they represent, placed at o, the impression produced by 

 the two drawings thus seen will be precisely the same as those which 

 would be produced on the right and left eye respectively by the object 

 itself seen at o. 



8. In the lenticular stereoscope invented by Sir David 

 Brewster, the form of the instrument to which the public in general 

 in all countries have given the preference, the visual rays pro- 

 ceeding from the two pictures are deflected and made to diverge 

 from the desired distance, by means of two eccentric double 

 convex lenses. 



These are formed by cutting a double^ convex lens A B c D (fig. 4), into 

 two semi-lenses BAD and BCD, in the 

 direction of a plane B D, passing through 

 the centre of the lens. The two eccentric 

 lenses are then cut out of these, so that 

 the diameters A E and c E shall be the 

 semi-diameters of the original lens. It 

 will be evident that a section of the origi- 

 nal lens, made by a plane passing through 

 A E c at right angles to its surface, will 

 have the form represented at ABC (fig. 5), 

 and consequently that the two eccentric 

 lenses A E and c E will have their thickest 

 part at E, and their thinnest at A and c. 

 While the geometrical centres of these 

 lenses are at o and o', their optical centres 

 are at the thickest point E of the radius. 



Now, suppose these two lenses to be set with their edges A and c towards 

 each other in two eye-holes whose distance apart is equal to that of the 

 eyes, and let two objects, p and p' (fig. 6), be placed before them at a 

 distance equal to their common focal length. According to the properties 

 of lenses already explained, pencils of 

 rays diverging from p and p', and passing -pig. 5. 



through the lenses, will be, after refrac- 

 tion, parallel respectively to lines drawn 

 from P and p', through the optical centres 

 E and E' of the lenses. Thus the visual 

 ray P p will, after refraction, issue in the 

 direction p L, and the ray p' p' will issue 

 in the direction p' R, so that the points p 

 and P' will be seen in the directions of 

 L p and R p' converging to the point o. 



Now, if P be a picture of an object as it appears to the left eye, and P' a 

 picture of it as it appears to the right eye, these two pictures will be 

 brought together at o by the refraction of the lenses, and the eyes will see 

 the combined pictures at o, exactly as they would see the object itself if it 

 were placed there. 



An advantage incidental to this arrangement is, that the 



141 



