= 21 = 



we now also supply it in a mount resembling that shown in Fig. 6 (ai) thereby 

 adapting it to other stands. Should the systems 3o and ai be employed on the 

 Mineralogical Stand III M or Stand XI -- which must be expressly so stated 

 in the order - - it is given a similar mount as the system 83; they can then 

 also be employed in combination with a nose-piece or sliding objective-changer. 

 Mounted in tJiis manner, the objectives produce a somewhat higher magnification. 



Objective a* (Fig. 7) consists of two 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 ocular, the magnification can be so greatly 

 varied that its power is approximately doubled by changing the position of the 

 index from to division 10 of the scale. 



In the year 1890 we constructed the Water Immersion D*. It facilitates the 

 investigation of living micro-organisms and objects suspended in water at medium 

 magnifications. 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 system tends to 

 restrict the greatest sharpness of the image to the central portion of the field. 



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

 good images with fresh as with sea water. 



For the investigation of fluids by means of the Appliances for the investi- 

 gation of Ultra-microscopical Particles, is the objective D * particularly adapted on 

 account of its great free-working distance. See prospectus relating to Appliances 

 for the investigation Ultra-microscopical Particles (M 164), p. 6. 



The Water Immersion Objective PI, an optical combination introduced by 

 us in 1898 under the designation of the Plankton-Searcher, serves essentially 

 the same purposes as the system just previously 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, and the depth of defini- 

 tion is considerable. The use of new kinds of glass, capable of resisting the 

 influence of water, has facilitated a well-nigh apochromatic correction. 



To facilitate the use of this objective, we supply a special glass cell 

 No. 2030 (glass slide with deep glass cylinder cemented thereon), suitable for 



use also with the paired objectives (Pl\ 



Glass cell for PI and 



Marks Codeword 

 3.50 Pajarita 



I, cna. 



