TRIPLE BACK COMBINATION 361 



fail- 1 -inch, but we have seen a better than this of about the same 

 period. 



Hugh Powell supplied a 1-inch of good quality, and a ^, J, , 

 iVinch fairly good. The apertures of the and the ^-inch are of 

 course very low. 



On the whole it may be said that the corrections are well 

 balanced in the lower lenses, and the apertures moderate ; but when 

 we conie to the higher powers it is the deficiency of aperture that 

 becomes so oppressively apparent. In 1844 Amici made a -f-inch 

 objective of 112 and brought it to England. It was understood 

 that extra dense flint w T as employed in the construction of this 

 objective ; but this is perishable ; and Mr. Ross altered slightly the 

 curves of Amici's construction^ and w r ith ordinary flint succeeded 

 in extending the aperture of a J-inch objective to 85, or '68 N.A., 

 and a T V inch objective to 135, or '93 N.A. Of this latter it was 

 affirmed that it was ' the largest angular pencil that could be passed 

 through a microscope object-glass.' 



In 1850 object-glasses were made with a triple back combination ; 

 these were attributed to Lister ; but it is also affirmed that they 



FIG. 314. An early FIG. 315. A triple- FIG. 316. A single- 



^-in. combination back combina- front combination 



by A. Ross. tion by Lister (or by Wenham. 

 Amici ?). 



were the previous device of Amici. It may well be a disputed point y 

 for it is quite certain that this device brought the dry achromatic 

 objective potentially to its highest perfection. The combination is 

 illustrated in fig. 315, and under the conditions of its construction it 

 may be well doubted if anything will ever surpass the results- 

 obtained by English opticians in achromatic objectives constructed 

 with this triple front, double middle, and triple back combinations,, 

 apart from the use of the new kinds of Jena glass. For the method 

 of computing the triple back, vide ' Journ. R. M. S.,' 1898, p. 160 etseq. 

 It may be noticed that Tully's objective had a triple back, but it was- 

 not the result of intended construction ; it was a fortunate combina- 

 tion the real value of which was neither understood nor appreciated, 

 and as a consequence its existence was evanescent. 



In this same year Wenham produced another modification of the 

 achromatic objective of considerable value, but more to the manu- 

 facturer than the user of the microscope. It consisted of a single 

 front ; the combination is seen in fig. 316, which, it will be seen, is a 

 simpler construction, but this did not affect in the least the price of 

 the objectives produced. Subsequently, however, the form was 



