438 



Notes. 



Fig. 121. 



the single front. The lens figured by Mr. Wenham in his 1869 

 paper has a single front, a double middle, and a triple back (fig. 122) ; 

 he states that the triple back was the invention of Mr. Lister in 1850. 

 He alters* this form of lens in 1873 to a single front, a triple 

 middle, and a single back, the whole of the correc- 

 tions being performed by the single flint in the 

 centre of the triplet. I have an example of this 

 kind of lens, and have seen many others, but can- 

 not say that they are better than the (1854) 1/6 

 with the single front and the three doublets. 

 Personally I am inclined to believe; that the triple back is older 

 than 1850, and it is doubtful if -it was the invention of Mr. Lister, 

 as he makes no mention of it. A good many improvements in ob- 

 jectives were undoubtedly due to Prof. Amici. 



The optical index which equals ■ '. .' ., reveals the true 



Initial power 



character of a lens, and shows the real advance made in object-glass 



construction. Example : — The 1/4 (before 1837) has an O.I. of 9*5, 



Fig. 122. 



and that of 1847 one of 9*8, only a slight difference ; this is accounted 

 for by the fact that the older lens is a true 1/4, while the later is 

 between a 1/5 and a 1/6 in power, so that the ratio of aperture to 

 focus is about the same in both. In the 1 in. lenses we have an 

 advance from 12*7 to 16*7, and then to 19*1. We may compare 

 these old lenses with the modern apochromatics, 1 in. O.I. 28, 1/2 O.I. 

 32, 1/4 O.I. 22, 1/6 O.T. 15, this last lens being not so greatly 

 different from the 1/6 of 1854. 

 Andrew Ross died in 1859. 







* Mon. Micr. Journ., ix. (1873) p. 157. 



