37O OBJECTIVES, EYE-PIECES, THE APERTOMETER 



It is another distinctive feature of the 3 mm. objective that it 

 has a triplex front ; thus Zeiss's 3 mm. (= inch focus) had the 

 errors from three unconnected lenses balanced by two triple backs, 

 i.e. nine lenses taken together, but it has since been constructed on a 

 different formula. 



The foci of the set of apochromatic lenses now made by Zeiss 

 are integral divisions of what may be termed a unit lens of 24 mm. ; 

 24 he chooses as a means of avoiding the inconveniences inseparable 

 from the use of the decimal system. 1 The unit lens is therefore a 

 little higher than 1 inch in power. In the series of dry lenses there 

 are two powers of the same aperture. Thus 24 mm. arid 16 mm., 

 corresponding to English 1 inch and f inch, each has an aperture of '3 ; 

 a 12 mm. and 8 mm. = English ^ inch and inch, have each an 

 aperture of '65 ; while a 6 mm. and a 4 mm. = J inch and ^ inch, 

 have both an aperture of '95. 



There are also water- immersions : a 2'5 mm. = -^ inch, with 

 N.A. T25, and two oil-immersions respectively 3 mm. and 2 mm. 

 = inch and T L inch, both being made either with 1'3 or 

 1-4 N.A. 



Apart from these, intended to be used for photographic purposes 

 without an eye-piece, is a 70 mm. = a 3-inch, also a 35 mm. or 

 1^-inch objective. 



With the exception of the 6 mm., 4mm., and 2'5 mm. objectives, 

 which have the screw-collar adjustment, this series have rigid mounts, 

 correction being secured by alteration of the tube-length. 



The performance of these lenses, as they are now made, is of the 

 very highest order. They present to the most experienced eye unsur- 

 passed images. They are corrected with a delicate perfection which 

 only this system, coupled with technical execution of the first order, 

 can possibly be made to produce. The optical polish, the centring, 

 the setting, and the brass work certainly have never been surpassed. 



It is a matter also worthy of note that Zeiss's apochromatic 

 series of objectives are true to their designations as powers. The 

 ^-inch is such, and not a ^-inch designated ^-inch. This was 

 equally true of the early achromatics. A. Ross produced a J-inch 

 under that name. One now before us, made fifty years ago, has an 

 initial power of 41 ; and that of ^ inch has an initial power of 21. 

 But modern achromatics of fair aperture are always greatly in. 

 excess of their designated power ; J are nearly ^-inch. A ^-inch 

 of 40 has an initial power of 25, and is a fL-inch ; -^,-inch 

 objectives are in reality ^-inch ; and J-inch objectives of 90 and 

 upwards have initial powers of 50 instead of 40, which they should 

 have, so that they are in reality iths ; some in fact by no means 

 uncommon have an initial power of 60, and are actually ^th-inch 

 objectives. 



This is explicable enough from the maker's point of view ; it is 

 far easier to put power into an object-glass than aperture. It is 



1 Although the foci of the lenses are expressed in integers, with the single excep- 

 tion of the water -immersion 2'5 mm., there are inconvenient decimal fractions in the 

 initial magnifying power of all the series except those of 2'5 and 2 mm. focus. 



