JOURNAL 



OF 



THE ROYAL INSTITUTION 



01' 



GREAT BRITAIN. 



ON THE FORMATION OF AN ARTIFICIAL COMPOUND 

 DISPERSIVE POWER IN A COMPOUND LENS, FOR 

 CORRECTING THE FRONT LENS OF A REFRACTING 

 TELESCOPE. 



BY PETER BARLOW, F.R.S. 



Cor. Mem. Inst. France, &c. 



general principle of correcting the front lens of an 

 object-glass for achromatism is very well understood; it 

 consists only in making the focal lengths of the two lenses pro- 

 portional to their dispersive powers. When this proportion 

 obtains, and the lenses are placed in contact, the spectrum 

 vanishes, and the object-glass is achromatic ; but when it does 

 not, a new spectrum is formed ; and by considering the com- 

 pound lens as a simple one, a new dispersive power is created, 

 and this lens thus formed may be employed to correct another 

 front lens. Also, as the artificial dispersive power thus created 

 may be made either greater or less, in almost any proportion, 

 than that belonging to either of the media by which the com- 

 pound lens is formed, the correction may be made at almost 

 any distance behind the front lens, and consequently the cor- 

 recting compound lens may be made proportionally less than 

 the front lens it is intended to correct. 



When the compound correcting lens is used in contact with 

 the front lens, it becomes the triple object-glass of Peter 



VOL. II. Auo. 1831. B 



