E. M. NELSON ON BINOCULARS. •*« 



an erector is placed either in front or behind the Wenham 

 prism. If it is placed in front of the Wenham prism, ^.e., 

 between the prism and the objective, we have at the back of 

 the objective mc CM, at the back of the erector lAlO oin, after 

 the cross-over, by the Wenham prism, ora lAIO. After the trans- 

 position and inversion at the eye-piece mc CM, the "ordinary 

 view" is restored; it is, therefore, orthostereoscopic. When 

 the erector is placed behind the Wenham prism, i.e., between 

 the prism and the eye-piece, we have at the back of the objec- 

 tive mc CM, after the cross-over by the prism CM mc. After the 

 erecting eye-pieces no change takes place ; CM mc is not the 

 " ordinary view," consequently the image is pseudostereoscopic. 



There are four conditions for orthostereoscopism, viz. : (1) 

 mc CM ; (2) nio OIAI ; (3) lAlO oui ; and (4) MC cm. (1) is the 

 " ordinary view ;" examples, Beck's simple microscope and 

 Tolles' compound ; (2] the "ordinary view " inverted ; example, 

 Stephenson's without the erecting prism ; (3) example, Wen- 

 ham, N'achet is precisely the same as (1), and (4) is the same 

 as (2). If w^e suppose a cube placed on a table it matters not 

 whether we stand in front of the table or go to the other side of 

 the table to view the cube ; in both cases we will obtain ortho- 

 stereoscopic effects. The same is true with the microscope ; it 

 matters not whether we stand behind or in front of a binocular. 

 If when standing heliincl a microscope we have the " ordinary 

 view " (1), in front of the microscope we get (3). Again, if 

 when hehind the microscope we have the " ordinary view " 

 invented (2), in. front we shall get (4). 



There are also four conditions for pseudostereoscopism, viz., 

 the non-" ordinary view " (5), cm MC, its inversion (6), oiu 

 lAlO (7), the front view of (5) OIAI uio, and (8) the front view 

 of (6) CM mc. 



We have, therefore, only two things to remember — 1st : The 

 "ordinary view," mcCM. 2nd: The non-" ordinary view," 

 cm M C ; the rest are only the inversions of these two and the 

 four front views. 



Taking the binoculars in their alphabetical order we find 

 that Abbe's is an inverting and transposing binocular eye-piece. 

 Without the semi-circular eye-piece caps* the arrangement at 



* Stereoscopism was obtained by dividing the eye spots, by Wenham. 

 "Journal E. M. S.," 1854, p. 4. 



JouRN. Q. M. C, Series II., No. 31. 4 



