ACHROMATIC OBJECT-GLASSES. 15 



of MM. Selligues and Chevalier ; and, by working on their plan, 

 he produced, in 1 S27, an Achromatic combination which surpassed 

 anything of the same kind that had been previously executed. From 

 that time, the superiority of the plan of combining three pairs* 

 of lenses (Fig. 10, i, 2, 3), which should be so adjusted as to 

 correct each other's errors, to the telescopic combinations first 

 adopted by Mr. Tulley, may be considered to have been completely 

 established ; and English opticians, working on this method, soon 

 rivalled the best productions of Continental skill. 



14. It was in this country that the next important improvements 

 originated ; these being the result of the theoretical investigations 

 of Mr. J. J. Lister, t which led him to the discovery of certain 

 properties in Achromatic combinations that had not been pre- 

 viously detected. Acting upon the rules which he laid down, 

 practical Opticians at once succeeded in producing combinations 

 far superior to any which had been previously executed, both in 

 wideness of aperture, flatness of field, and completeness of correc- 

 tion ; and continued progress has been since made in the same 

 direction, by the like combination of theoretical acumen with 

 manipulative skill. For the subsequent investigations of Mr. 

 Lister have led him to suggest new combinations, which have been 

 speedily carried into practical execution ; and there is good reason 

 to believe that the limit of perfection has now been nearly reached, 

 since almost everything which seems theoretically possible has been 

 actually accomplished. The most perfect Objectives at present in 

 use for high powers, consist of as many as eight distinct lenses ; 

 the front and back being triplet combinations, with a doublet 

 between. In this manner an Angular Aperture of no less than 

 170° has been obtained with an Objective of l-12th inch focus ; and 

 it is obvious that as an increase of divergence of no more than 10° 

 Avould bring the extreme rays into a straight line with each other, 

 they would not enter the lens at all ; so that no further enlargement 

 of the aperture can be practically useful. — A principle of con- 

 struction for Objectives of high power, first devised by Amici, has 

 of late years been carried out by M. Hartnack (the successor of 

 Oberhauser) of Paris, as also by MM. Nachet, with great success ; 

 that, namely, which is known as the immersion system. This con- 

 sists in the interposition of a drop of water between the front lens 

 of the objective and either the object itself or its covering-glass ; 

 so that the rays which leave it to enter the objective do not pass 

 through air, but through water. It is easily shown that the 

 loss of light dependent on the reflection of a portion of the oblique 



* Some Opticians, however, prefer to place a single (uncorrected) plano- 

 convex lens in the front of the objective, and to over-correct the com- 

 binations behind it : this method, which is believed by the Author, on 

 the authority of Mr. Thomas Ross, to have been followed by Amici, seems 

 particularly adapted to Objectives of about l-4th inch focus and of mode- 

 rate angular aperture. 



t See his Memoir in the " Philosophical Transactions," for 1829. 



