422 Mr. Coddington on the Improvement 



all other persons have been satisfied with endeavouring to show, 

 as distinctly as possible, one individual point of an object, 

 trusting that the rest would follow of itself, or giving up, as 

 hopeless, the idea of producing a good and large field of view. 



To those who have studied the construction of the compound 

 microscope, an analogy presents itself, very naturally, between 

 that instrument and the telescope. In each there is an image 

 fonned, which is seen through one or more lenses, constituting 

 what is technically termed in the former case, the bodi/, in the 

 latter the et/e-piece. 



The progress of these instruments has been curiously similar 

 in some respects. The first step of anj^ consequence in the case 

 of the telescope, was Huyghens's eye-piece, which, besides the 

 merit supposed by its author, of diminishing the errors arising 

 from aberration, had one, much more imjxtrtant, which he did 

 not contemplate, the correction of the coloured fringes, seen 

 about every point of the image, except that precisely in the 

 centime*. Ramsden then succeeded in making an eye-piece, which 

 gives a flat field of view, when that point is particularly im- 

 portant, and finally, the instrument has been made perfect, by 

 substituting for the simple object glass, an achromatic and 

 aplauatic combination of lenses. In the compound microscope, 

 the first point, (the correction of the coloured fringes,) has been 

 completely attained ; on the .second, much labour has been 

 bestowed by practical opticians, but with little success ; the 

 third has lately occupied some of the most distinguished theorists 

 and artisans, who have been eminently successful, but the 



■ • This is the more remarkable, as this point was completely attained at once, whereas 

 the other was long but half gained. It is but of late years, that even an approximation 

 to the proper forms of the lenses has been made in the achromatic eye-piece. 



