ANGULAR APERTURE. [ 47 ] ANGULAR APERTURE. 



the rays will be refracted so as not to enter 

 the oDJect-glass, whilst the other set will 

 gain admission ; thus the two parts, which 

 have differently refracted the rays, will be- 

 come distinct. If the markings were more 



Fig. 20. 



delicate, or the difference between the re- 

 fractive power of the two portions of the 

 valve were less than that represented in 

 fig. 20, both sets would enter the object- 

 glass. But on rendering the light still more 

 oblique, one set would be again excluded, 

 from being refracted out of the field. Hence 

 it is evident why the angular aperture of 

 the object-glass must be larger, as the 

 markings are finer, or the difference between 

 the refractive power of the two portions of 

 tissue is less ; because the obliquity of the 

 light requisite to cause the exclusion of one 

 set of the rays will be very great, and the 

 other set will be too oblique to enter the 

 object-glass, unless it be of correspondingly 

 large aperture. 



In this way, we were wont to account for 

 the action of large angles of aperture and 

 oblique light in rendering visible the finer 

 markings of objects. 



But the entirely new researches of Pro- 

 fessor Abbe of Jena, confirmed by others, 

 especially Crisp, Stepheuson, and Nageli and 

 Schwendener, have entirely altered the 

 views upon this subject. 



According to Abbe's experiments and 

 mathematical demonstrations, the resolution 

 of the more minute structural peculiarities 

 of objects those of less size than ^Yo f 

 an inch is produced by the superposition 

 of two images, one of which principally 

 contains the outlines of the- object, and is 

 produced according to the ordinary laws of 

 dioptrics ; while the other, which yields 

 the finer structural details, arises from the 

 diffraction (or an equivalent deflection pro- 



duced by refraction or reflection) of the rays 

 within the object, and their interference at 

 the upper focal plane of the object-glass, 

 where they produce distinct interference- 

 images. Unless at least one of the diffrac- 

 ted beams is admitted by the object-glass, 

 together with either another beam or the 

 direct incident pencil, no indication of struc- 

 ture is visible in the microscopical image. 

 And the angle of aperture of an object-glass 

 represents a specific property, entirely dis- 

 tinct from the magnifying and other pro- 

 perties of the object-glass. 



If the interference-images are stopped off, 

 or if the angular aperture of the object-glass 

 is too small, the corresponding details dis- 

 appear entirely from the microscopic images. 



Abbe holds also, that structures of this 

 minute kind which an object-glass with 

 direct illumination does not render visible, 

 are also not visible when the object is in- 

 clined at any angle whatever to the axis of 

 the microscope; even when, while lying 

 perpendicularly to its axis, they are perfectly 

 resolved by oblique light. 



An important result of these views is, 

 that minute structural details are not, as a 

 rule, imaged by the microscope dioptric-ally, 

 in accordance with the real nature of the ob- 

 ject, and cannot be interpreted as morpho- 

 logical but only as physical characters : not 

 as images of material forms, but as signs of 

 material differences in the nature of the 

 particles composing the object, so that no- 

 thing more can safely be inferred from the 

 image aspresentedto the eye than the presence 

 in the object of such structural peculiarities 

 as will produce the particular diffraction- 

 phenomena on which the images depend 

 (Abbe). 



In regard to the action of oblique light 

 and angular aperture in relation to colour : 

 neither oblique light nor large angular aper- 

 ture possesses any power of rendering co- 

 loured transparent objects more distinct; 

 and markings, when arising from the pre- 

 sence of pigment, are perfectly visible under 

 an object-glass of small aperture, and the 

 ordinary light of the mirror. 



When objects are examined under im- 

 mersion-lenses, they appear more brilliantly 

 illuminated, especially in regard to the finer 

 details. This is partly to be attributed to 

 the diminution of the loss of light from re- 

 flection at the surfaces of the cover and the 

 under surface of the object-glass; but mainly 

 to the circumstance that a larger number 

 of oblique rays are enabled to enter the 



