112 THE MICROSCOPIST. 



Power of definition depends, in a great measure, upon the 

 angle of aperture of the object-glass. A deficiency of angular 

 aperture is shown by a want of light, producing unsatisfactory 

 vision, which is rather increased than ameliorated by augment- 

 ing the intensity of the artificial illumination ; by an incapa- 

 city of showing lined objects, except such as are of the lowest 

 class ; and by giving very large spurious discs with artificial 

 stars; also by showing easy test objects with the lines faint, 

 while the spaces between them are darker and more opaque 

 than they ought to be. 



When the aberrations are properly corrected; and the angle 

 of aperture considerable, the lines on test objects become fine, 

 sharp, and dark, and the spaces between them bright, pro- 

 vided the illumination has. been properly conducted; they 

 moreover become visible in a very faint light; the outline and 

 the lines are seen at once; and the spurious discs of all bril- 

 liant points are very sharp and small. 



In order to explain more fully what is meant by angular 

 aperture, let A and a, Figs. 30 and 31, represent two objects, 

 in all respects alike ; and suppose B, B, and 5, b, to be two 

 object-glasses of equal focal length ; the former a single lens, 

 of the best construction, such as was used in the old compound 

 microscope, and the latter a lens of the newest form, termed 

 an achromatic. Now these object-glasses will form their re- 

 spective images at I and i, and they will be of equal dimen- 

 sions. But if the number of rays proceeding from A and 

 falling upon the single lens B, B, is not enough, when col- 

 lected at I, sufficiently to stimulate the eye, any minute pore, 

 stria, or other marking at A, will not be rendered visible ; 

 while from the increase of aperture in 6, 6, allowing much 

 more light to be transmitted, every mark at a, will be repre- 

 sented at i t and the eye being powerfully acted on by the 

 increase of light, will be highly sensible of it. 



