15 



Our lowest power objectives, viz: a<j, EI, 82 and as, are simple achromatic 

 'lenses, so mounted that, notwithstanding their great focal length, the body of 

 the microscope remains at its ordinary elevation during observation. For this 

 purpose lenses ao and ai have their threads so placed that, when screwed into the 

 tube, the lens is inside the body (see HI, Fig. 5), and they cannot therefore be 

 used with a revolving nose-piece or other objective-changer. Though 

 the former is primarily intended for use with our binocular microscopes, we now 

 also supply it in a mount resembling that shown in Fig. 5 (aO thereby adapting 

 it to other stands, except the Mineralogical Stands l b and IX b . 



Objective a* (Fig. (>) consists of t\vo achromatic lenses combined after an 

 original formula of our own computation. By means of a ring RR, rotating like 

 a correction -collar, the lenses may be brought closer together or separated, 

 whereby, in combination with an eye-piece, the magnification can be so greatly 

 varied that its power is doubled by changing the position of the index, from 

 to division 10 of the scale. 



At the suggestion of Mr. BRATUSCHECK we have in 1890 added to the 

 water immersion series a lens giving a large free working distance but 

 having a relatively small aperture, viz: Objective D*. Its special purpose is 

 to facilitate the examination of living organisms floating in water-troughs at 

 intermediate magnifications (200 to 500 diameters). Its great working distance 

 gives to the microscopist a relatively wide range in following the motions of 

 such creatures through both the upper and lower strata of the water. The 

 peculiar construction of the lens tends to restrict the greatest sharpness of the 

 image to the central portion of the field. 



The lens may be used with or without cover-glass and gives equally 

 good images with fresh as with sea water. 



The water immersion objective PI, an optical combination devised by 

 Dr. H. HARTING and introduced by us in 1898 under the designation of the 

 Plankton Searcher, serves essentially the same purposes as the lens just pre- 

 viously described. Owing to its long focus this objective yields only low 

 magnifications, and though its aperture is 0.11, it gives the very large working 

 distance of about 36 mm. In combination with different eye -pieces its optical 

 qualities (magnification and diameter of field) differ but slightly from those of 

 the lens aa, and the depth of vision is equally considerable in both forms of 

 objective. The use of new kinds of glass, capable of resisting the influence of 

 water, in the construction of these lenses has facilitated a well-nigh apo- 

 chromatic correction. 



