MICROSCOPIC ILLUSTRATIONS. 251 



aberration ; secondly, those which give evidence of the 

 presence of a large angle of aperture ; and thirdly, those 

 which shew the union of the above properties, and con- 

 sequently the greater or less approximation of an instru- 

 ment to its most perfect condition. 



3. Chromatic aberration is rendered sensible by almost 

 any transparent object, when the light falls upon it obliquely; 

 but more especially by such as are not transparent, but only 

 illuminated by intercepted light, of which a very good 

 example may be seen in a piece of fine wire sieve, treated 

 like a diaphanous object, also in a thin plate of metal, 

 perforated by very small holes. The various colours are 

 seen according to the order of their refrangibility, by put- 

 ting the object both without and within the focus, as well 

 as by viewing it at the focal point; all brilliant opaque 

 objects also exhibit chromatic aberration strongly, when 

 managed in the same way. 



4. Spherical aberration is most sensibly felt in viewing 

 opaque objects, especially if of the brilliant class ; it shews 

 itself in a variety of ways : first, as a diffused nebulosity 

 over the whole field of view ; secondly, as a confined 

 nebulosity, extending only to a certain distance from the 

 object; and thirdly, in a want of sharpness and decision 

 in the outline caused by a penumbra or double image, 

 which can never be made to lap perfectly over the stronger 

 or true one. Aplanatism, or a destitution of the aberration 

 of sphericity, is evinced by the absence of these appear- 

 ances, and by the vanishing of the image immediately the 

 object is put out of focus either way. 



5. A deficiency of angular aperture is shewn by a want 

 of light, producing unsatisfactory vision, which is rather 

 increased than ameliorated, by augmenting the intensity 



s2 



