374 OBJECTIVES, EYE-PIECES, THE APERTOMETER 



diatom one indeed that was considered critical until that with the 

 apochromatic was seen. But in comparison it is dull and yellowish. 

 From which it follows that an exceptionally fine achromatic -j^-inch 

 of 60 or '5 N.A. will not suffer comparison of the image it yields 

 with that of an apochromatic Vinch of "65 N.A. 



Speaking generally on the whole question, then, it would be the 

 utmost folly for histologists or opticians to shut their eyes to the 

 magnificent character of the series of dry apochromatics of Zeiss, 

 ranging from 1 inch (24 mm.) to j? inch (4 mm. '95 N.A.). They 

 are the most perfect and efficient series of objectives ever placed in 

 the hands of the worker ; and, unless English lenses on a truly 

 apochromatic principle and equal quality are produced, it must be 

 to the detriment of either the opticians or the workers of this 

 country. 



Nor need it be supposed that the production of objectives 

 approximate to these must be costly ; great steps have been taken 

 lately in the reduction of their cost. The manufacture of the Jena 

 glass has indeed wrought an entire change in the character of 

 objectives now produced ; and although the very finest and most 

 costly apochromatics having nuorite used in their construction still 

 hold an unrivalled position, yet the new glass admits of corrections 

 so nearly perfect that some stronger word than achromatic appeared 

 to be needed, and the word semi -apochromatic has crept in and 

 undoubtedly designates a most valuable and far from costly set of 

 lenses of all powers. It is Leitz, of Wetzlar, that has first and 

 efficiently attacked this problem and provided the student whose 

 means are limited with objectives of a very high class, and which 

 come remarkably near to the best apochromatics. We would 

 specially call attention (wholly in the interests of students) to No. 3 

 (j-inch N.A. 0*28) at a cost of 15s. No. 5 is an equally valuable 

 and admirable objective which is a J-inch O77 N.A., the price of 

 which is 25s., and it comes so near to an apochromatic as to require 

 expert judgment to discover that it is not. He also makes a dry 



-iV incn N - A - O' 8 ? and a <% 6 of >82 N - A - at a cost of SI-' which 

 is a very low price for so good a piece of optical work. Also an 

 oil-immersion T ^-inch N.A. T30 is sold for 31. 15s. This glass is 

 corrected for the long tube, and a similar ^ih N.A. 1*30 for 

 51. resolves secondary diatom structure well, and it is hardly dis- 

 tinguishable from an apochromatic lens ; and we can attest, from 

 personal investigation, the value of each of these, which are only 

 selections from a considerable series, all of which w r e have found 

 to be reliable, and, when examined in numbers, very few indeed 

 are below the standard quality. But such work is so much needed 

 that it is not likely that, with the glass accessible to all, it will 

 remain the peculiarity of one maker ; hence we find that Reichert 

 follows Leitz so closely in quality and price that it is not easy to 

 distinguish the semi-apochromats of one maker from the other. 

 Reichert's No. 3 (4-inch N.A. O30) is 17s., his 7A (an admirable 

 lens) i-inch N.A. 0*87 is II. 16s. He makes a high-class oil- 

 immersion T L-inch N.A. 1*30 for SI. And of apochromatic lenses 

 he makes a 2-inch N.A. O30 and a ^-inch N.A. O95 for 4. each, 



