76 Modern Microscopy 



scopist, will with a higher power namely, the ^-inch 

 show him some of the minuter detail which sooner or later 

 he will wish to make himself acquainted with. If more 

 object-glasses than these be required, we should recommend 

 the i-inch as an intermediate between the 1-inch and the 

 ^-inch, and for a higher power a T V-inch oil immersion 

 objective should be added. It is well to buy only such 

 low-power objectives as have double combinations. Some 

 of the cheaper ones consist of two or three lenses balsamed 

 together in one combination only ; with these there is an 

 insufficiency of aperture, and good definition and flatness 

 of field cannot be obtained. All the best low-power lenses 

 are constructed with two pairs or more of lenses set a little 

 distance apart, and can be readily recognised. Of the 

 apochromatic series of Zeiss our choice would be the 

 24, 12, 6, and 3 (1-4 N.A.) millimetre objectives if for the 

 English tube-length, or the 16, 4, and 2 (1-4 N.A.) if for 

 the Continental tube-length. The 12 and 3 millimetres are 

 considered the finest of the series. Messrs. Powell and 

 Lealand supply a -j^-inch apochromatic oil immersion 

 objective of 1/4 N.A. which enjoys special favour, the price 

 of which is only W. Of achromatics that approach the 

 apochromatics in performance the 8 millimetres by Beichert 

 and his 4 millimetres (1*3 N.A.) homogeneous immersion 

 are very fine, especially the latter. We have worked with 

 both of these, and they have given most excellent results. 

 The No. 6 and T V-inch of the same maker are also extremely 

 good. Leitz, of Wetzlar, is the maker of a very good series 

 of achromatic objectives also, and his iV-inch, 3 15s., 

 and T V-inch, 5, both for oil immersion, are excellent. As 

 previously remarked, competition has caused all the makers 

 to bring their objectives to a high level of perfection, and 

 the novice will be quite safe in equipping himself with those 

 of any of the leading English opticians. The productions 

 of American opticians are but little known in England, yet 

 they are said in many instances by competent judges to be 

 of exceptional quality, Messrs. Bausch and Lomb and the 



