586 Transactions of the Society. 



An examination of the first group of lenses in the above table 

 shows the fairly high position taken by non-achromatic lenses; 

 this may be accounted for by the absence of all superamplification, 

 for if they had been used with an eye-piece a very different result 

 would have been obtained. 



It will be noticed that the \ inch, both single and double, wins 

 the greatest number of marks, and it is here where the high water- 

 mark was reached in pre-achromatic days. In the higher powers 

 the doublets surpass the singles, and the Chevalier doublet is the 

 best, for the Chevalier doublet £ is one above the single, and the 

 -jJg is two above the Wollaston doublet of nearly the same power, 

 and the ^o is not one behind the Pritchard ^ although it is a 

 much higher power. 



Passing on to the second group of lenses, viz. early Tulley 

 achromatics,* we see that the lower powers have a fairly large J., 

 and come out better than one would have expected. "With the B 

 eye-piece, however, the 1^ inch loses 2, but the t 9 q, the best of the 

 series, only 1. 



The combinations lose 2 when the front is put on. The back 

 of the last combination is fairly good with 9 ■ 8 marks, but the 

 § and \ combination is especially bad, the last only obtaining 

 3 ■ 7 marks. 



Three examples extracted from the table in my previous paper 

 are inserted at the end for purposes of comparison. The achro- 

 matic 1^ (1860) corresponds very nearly in power with the Tulley, 

 but its O.I. is 8 more, and it scores 3 more marks. The semi- 

 apochromatic 1 has 10 more O.I., and wins besides 10 more marks 

 than the Tulley, and only loses ■ 6 with the B eye-piece. 



This table therefore shows at a glance the advance made in the 

 construction of Microscope lenses since 1830. 



These Tulley combinations are interesting as they are very 

 early, if not the earliest examples of separating achromatic lenses, 

 the construction of which passed on from Tulley to Smith. It 

 will be remembered that it was Smith who, in 1841, made the 

 separating lenses for the Microscope purchased by the Society, 

 which are still in our Cabinet. Another interesting feature is 

 that the gauge of Tulley's screw is very nearly the same as that 

 of the Society's screw, so that Tulley's objectives will enter and 

 screw into our nose-piece, but Tulley's nose-piece is a trifle too 

 small for the Society's plug to enter. 



* For an account of these lenses see Journ. E.M.S., 1902, p. 16. The 1 J, ^, and 

 the back lenses of the combinations are similar to the Gdring-Tulley triple of 1824. 

 The fronts of the combinations are higher powers of the same form. The front lens 

 of the last combination was out of adjustment, therefore could not be examined. 



