ON APERTURE AND DEFINITION" OF MICROSCOPE OBJECT GLASS. 455 



of observing many useful eifects, particularly when ''positive" 

 images only are formed. But the more usual form of oblique 

 illumination, by placing the mirror out of axis, tests to a greater 

 extent the advantage of large aperture in all cases where the object 

 to be resolved contains very minute structural elements, especially 

 when arranged in equidistant lines or points placed either in 

 parallel position or at particular angles to each other. The effect of 

 this oblique illumination is directly proportionate to the amount of 

 inclination of image-forming pencils to the axis of the microscope, 

 and, of course, as only one side of the marginal zone of front lens 

 is in operation, and, therefore, fewer images are formed, there is less 

 overlapping and confusion of outlines. 



In respect to the third point — the admission of pencils of light 

 diffracted in passing through the object — ^large aperture is so 

 essential that it may be said to add (or at least to permit) an 

 entirely new function to the objective. According to Professor 

 Abbe the capacity of "resolving" all minute details is dependent 

 upon the formation of "positive" images by the combination of 

 two or more diffraction pencils, caused by structural peculiarities of 

 the object. These diffraction pencils enter the objective in virtue 

 of its large aperture, and form a diffraction image (or positive 

 image) independently of the absorption image (or negative image), 

 which latter is formed according to the dioptric law by which 

 homocentric pencils proceeding from a focal point are re-united 

 in its conj ugate focal point. 



But neither is the negative nor the positive image formed with 

 large pencils filling the whole front of the lens system. Each 

 passes independently through different zones of the lens, and 

 changes its position as the illumination is changed. Unless they 

 fall together on the same focal plane, and are accurately super- 

 imposed, these negative and positive images will appear in front of, 

 or behind, or beside, each other, and, therefore, the "resolving" 

 power does not necessarily become additional " defining " power. 

 Again, the definition of each image may not be much deteriorated 

 by faulty focussing function or colour dispersion — when the pencils 



