.^6 fi. M. NELSON ON BINOCULARS. 



parallel to this page, and in such a manner that the left eye can 

 only see the riglit-hand picture and the right eye the left-hand 

 picture. 



Now are these figures " pei'spective " drawings, or are they 

 " parallactic displacements " ? They are called perspective 

 drawings, but in reality they are only parallactically displaced. 

 There is no foreshortening, the base is a square, and the top is 

 a square displaced to one side, and lines ruled joining the 

 corners. If you consider the centre of the truncated pyramid, 

 the focal plane, the base is parallactically displaced one way, and 

 the truncated top the other. 



These drawings, which are exact copies of those given in the 

 text-books (originally French), are exaggerated representations 

 of the images seen by each eye. A cube illustrates the same 

 effect, and it matters not whether you draw it in true perspec- 

 tive, the edges of the side of the cnl)e beino- portions of Hues 

 drawn to the vanishing 

 point (Fig. 8), or draw it 

 parallactically displaced 

 with the edges of the side 

 of the cube parallel to 

 one another (Fig. 9). If the drawings of the cubes are exag- 

 gerated to anything like the same extent as those of the 

 truncated pyramid they will not coalesce ; the resultant picture 

 will be merely a super-position of two dissimilar cubes. It is 

 the latei'al displacement which is the sole and important point, 

 and it makes no difference whether that is obtained by true per- 

 spective or by parallactic displacement, because the eye cannot 

 distinguish between them, the displacement at its greatest being 

 only 8°, perspective foreshortening is impossible.* 



With regard to microscopical stereoscopism, if the image of a 

 plane object, such as ruled squares, suffered perspective fore- 

 shortening by reason of aperture, different zones of the objec- 

 tive would yield different images, and the resultant picture 

 would be confused. Therefore it goes without saying that a 

 microscope image of even such an elementary object would be 

 simply an impossibility. This Prof. Abbe ably points out. 



But with regard to depth, the depth of vision is so minute 



* Perspective foreshortening is as cos 9 : 1 ; therefore for 8'^ it would 

 be in the proportion of 99 to 100. 



