( 203 ) 



IV. — On the Conditions of Ortlwscopie and Pseudoscopic Effects 

 in the Binocular Microscope. 



By Professor E. Abbe, Hon. F.R.M.S.f 



{Read 12th January, 1881.) 



In a paper which appeared in the September issue of the ' Zeitschrift 

 fiir Mikroskopie,' | the author briefly pointed out a principle by 

 which the well-known diversity of stereoscopic effects in the 

 binocular Microscope — orthoscopic or pseudoscopic vision — is 

 brought to a general and very simple expression, defining the 

 character of stereoscopic action in binocular arrangements of any 

 kind, independently of the composition of the optical apparatus. 



The principle in question is based on the consideration of the 

 so-called " Kamsden circle " at the eye-point of the ocular, which 

 is a reduced image of the objective's clear aperture, and represents 

 the common cross-section of all delineating pencils at their emer- 

 gence from the ocular. As in the monocular Microscope all rays 

 gathered by the circular aperture of the objective are transmitted 

 through the system, the Eamsden circle is a true circular figure 

 which is conjugate, point for point, to the clear aperture. Stereo- 

 scopic vision, however, requires that the delineating pencils shall be 

 so divided that one portion of the admitted cone of light is con- 

 ducted to one eye and another portion to the other eye. Hence 

 the Eamsden circles above the eye-pieces of a stereoscopic Micro- 

 scope must always be reduced to two dissimilar figures, representing 

 dissimilar parts of the full circle of a monocular Microscope ; for 

 different (e. g. opposite) portions of the incident cone obviously 

 correspond to different (opposite) portions of the emerging pencil. 

 If now the division of the pencils, by whatever optical means it 

 may be effected, is effected in a symmetrical way (which is usually 

 the case in binocular arrangements), their cross-sections above the 

 oculars must be reduced to semicircles, representing one of these 

 two arrangements or P : — 



Fig. 36. 







This being premised, my proposition is : — 



The only condition necessary for orthoscopic effect in any 



t The original paper is written by Prof. Abbe in English. — Ed. 



j " Besehreibnng eiues neuen stereoskopischen Oculars, nebst allgeraeinen 

 Bemerkuugen iiber die Bedingungeu mikro-stereoskopischer Beobachtung," 

 Zeitschr, f. Mikr., ii. (1880) p. 207. See also i7ifra, "Microscopy." 



