On the Mode of Vision with Wide Apertures. By Prof. Abbe. 25 



pencils a m and yQ m to the image by tlie axial pencils a a and 

 /3 a, or to the image by other oblique pencils (say of opposite 

 obliquity), we have dissimilar images. But this dissimilarity 

 relating solely to the projection of successive layers, and being 

 nothing else but different parallactic displacement of successive 

 layers, cannot be effective in microscopic vision unless these images 

 are produced by different portions of the aperture separately, that 

 is, if the effective pencils (or the effective portions of the aperture) 

 are separated, and the one conducted to one image and the other 

 to another image, as is done by the various arrangements for 

 stereoscopic vision. As long as various portions of the aperture 

 are effective at the same time, producing one image, we have only 

 an increase of the dissipation circles at those planes which are 

 not exactly focused, and a reduction consequently of the depth of 

 distinct vision. We have no "all-round vision'' because vision 

 ceases as soon as the " all-round " becomes effective. 



The result of the whole consideration therefore is : — (1) In a 

 well-corrected (or aplanatic) objective the images of a flat object 

 by pencils of different obliquity are always strictly similar. The 

 obliquity of the rays at the object does not produce any difference 

 of perspective, as it does in ordinary vision, or when the same 

 object is observed by a Microscope in an oblique direction. The 

 Microscope therefore does not delineate solid objects perspectively, 

 and has no capacity of all-round vision, either as a drawback or a 

 benefit. 



(2) The images of solid objects arise from the projection of 

 their successive layers in perfect similarity, however large the 

 aperture may be (refraction of the rays by structural parts within 

 the layers disregarded). As long as the depth of the object is 

 within the limits of the depth of vision corresponding to the 

 aperture and amplification in use, we obtain a distinct parallel 

 projection of all successive layers on one common plane perpen- 

 dicular to the axis of the Microscope (a regular ground plan), either 

 strictly orthogonal (fig. 7) when the 

 delineating pencils, narrow or wide, are Fig. 7. 



axial, or with a certain obliquity of pro- 



axiai, ox Willi a ueiiain uuiiquii^ ui piu- , ; j \ / — -, — j 



jection if these pencils (i. e. the axes or j j j j /__/_/ 



principal rays of the pencils) are inclined 



to the axis of the Microscope. If the depth of the preparation is 

 greater than the depth of tolerably distinct vision, this jDrojection 

 must become indistinct, because the layers above or below the range 

 of distinct vision give rise to broad dissipation circles confounding 

 with the distinct portion of the image. Since the depth of vision, 

 other circumstances being equal, decreases with increasing aperture, 

 good " definition " of wide apertures is confined to thinner objects 

 than good definition of narrow apertures. 



