INTRODUCTORY RKMARKS. 45 



therefore mounted his glasses with the flat side of the 

 flint lens foremost, and he made the whole still more 

 valuable by the superiority of his workmanship. It was 

 not, however, until these object-glasses arrived in this 

 country, and their aperture was increased by Dr. Goring, 

 that their full effect was brought out. 



In the early productions of M. Chevalier, the foci of 

 the different pairs of lenses composing an object-glass were 

 of unequal lengths ; the anterior pair having the shortest, 

 and the posterior the longest focus. Some of these sets 

 were admirably corrected ; and it is much to be regretted 

 that on account of the comparative difficulty there is in 

 this plan of constructing them, he should have been in- 

 duced to make the pairs of equal foci, by doing which the 

 maximum of angular aperture cannot be obtained together 

 with a fine definition. When we consider the errors which 

 are to be corrected in an object-glass, in order that it may 

 produce a magnified representation of an object suffi- 

 ciently perfect for it to undergo a second amplification by 

 the eye-piece ; and when we consider further that many 

 errors, which under ordinary circumstances are barely 

 appreciable, become very evident as soon as some prin- 

 cipal error is subdued, it is not surprising that the achro- 

 matic construction has been so long progressing towards 

 its present state of perfection. To say nothing of sphe- 

 rical and chromatic aberration, before mentioned, it is 

 no easy task to correct for the central and oblique pencils, 

 so as to obtain a moderately flat field ; the more especially 

 when we consider how restricted we are to certain forms 



