96 THEORY OF THE MICROSCOPE. 



large that its diffraction-effect does not appreciably prejudice the 

 sharpness of the image, the magnitude of the angle of aperture 

 is without influence upon the representation of the contours of 

 microscopic objects, that is, the boundaries between the unequally 

 transparent parts in the field of view, when these parts are not 

 less than about 10 mic. 



(2.) On the other hand, as soon as the objects become smaller 

 in their details than the stated limit of minuteness, a noticeable 

 difference constantly appears in favour of the larger angle of 

 aperture. At the same time it is immaterial whether the details 

 in question are produced by unevenness of the surface, or by 

 differences in the transparency of the substance. 



(3.) The smaller the linear dimensions of the details in question, 

 the larger must be the angle of aperture of the objective, if they 

 are to be perceptible with any particular kind of illumination (for 

 instance, either entirely central, or as oblique as possible), inde- 

 pendently of the greater or less relief and sharpness of the details 

 of the object, and also of the focal length and magnifying power 

 of the objective. 



(4) When the details of the object take the form of striae, 

 systems of lines, &c., the same angular aperture invariably exhibits 

 with oblique illumination perceptibly finer details than with 

 central illumination, whether the constitution of the object admits 

 of the possibility of shadow effects, or completely excludes them. 



(5.) A structure of this kind, which a particular objective does 

 not resolve with direct (axial) illumination, is still invisible if the 

 plane of the object is inclined at any angle whatever to the axis 

 of the Microscope, though it may be completely resolved by 

 oblique light when lying at right angles to the axis. When the 

 plane of the object is inclined to the axis of the Microscope, 

 resolution is, however, at once obtained if the incident pencil is 

 directed perpendicular to this plane. Consequently, the increased 

 effect of oblique illumination depends exclusively on the obliquity 

 to the axis of the Microscope, and not on the obliquity to the 

 plane of the object. 



The angle of aperture therefore represents a specific power which 

 is not dependent on the other properties of the objective, nor on 

 its magnifying power, the power of rendering perceptible in the 

 microscopic image fine structural details, which, at a lesser incli- 

 nation of the incident rays, remain invisible. This may be called 



