ABERRATIONS EFFACED. 



To appreciate the circumstances in which these great difficulties 

 have consisted, it will be necessary that the reader should revert 

 to our Tract upon " Optical Images," 39 et seq. It is there shown, 

 that when an object is placed before a convex lens, the image of it 

 which is produced, is not in any case a faithful copy of the object. 

 , In the first place, each portion of the lens, proceeding from its 

 centre to its borders, produces a separate image ; this series of 

 images, being ranged at different distances from the lens : when 

 these images are looked at, as they would be, for example, with 

 the eye-glass of the microscope, they are seen projected one upon 

 another, and being slightly different in their magnitudes, a con- 

 fusion of outline and lineaments ensues, so that the object appears 

 as though it were viewed through a mist. 



This sort of indistinctness, called spherical aberration, has been 

 fully explained in our Tract upon " Optical Images," and the 

 general principles, by which its effects may be more or less miti- 

 gated, have been there explained. 



It has been in the diminution, if not entire extinction, of this 

 cause of indistinctness, by the happy adaptation of the curvatures 

 of the lenticular surfaces entering into the optical combinations 

 which form the microscope, that the address and genius of the 

 practical opticians has been chiefly manifested ; and if it cannot 

 be stated, with strict truth, that all the effects of spherical aber- 

 ration have been effaced in the best instruments now placed at 

 the disposition of the observer, it may, at all events, be safely 

 affirmed, that they exist in so small a degree as to offer no serious 

 impediment to his researches. 



But independently of this source of indistinctness, there is 

 another which has also been fully explained in our Tract upon 

 " Optical Images," 39. 



Light is a compound principle, consisting of several elements, 

 differing in colour and also in refrangibility, the consequence of 

 which is, that when an object is placed before a convex lens, it 

 is not one image which is formed of it, but a series of images, 

 varying in colour, from a violet or blue, through all the tints of 

 the rainbow, to a red ; these images are placed at slightly dif- 

 ferent distances from the lens, and when viewed through the eye- 

 glass, would be projected one upon the other, and being of slightly 

 different magnitudes, the consequence of such projection would 

 be, that their outlines, and those of all their parts, would be 

 more or less fringed with iridescent colours, an effect which, 

 it is needless to say, would destroy the distinctness of the 

 lineaments. 



12. The principle upon which this chromatic aberration is 

 counteracted, has been fully explained in our Tract upon "Optical 



9 



