THE OPTICAL EQUIPMENT 33 



1. A Planar or micro- Summar, or a lens of similar con- 

 struction (preferably one of rather long focus, either the 75-mm. 

 or 100-mm.), for photographing large objects or sections. 



2. A If- or 1-inch achromatic, or 35-mm. apochromat. 



3. A two-thirds-inch or 16-mm. apochromat. 



4. A one-third-inch or 8-mm. apochromat. 



5. A one-sixth-inch or 4-mm. apochromat with adjustment 

 collar allowing of correction for thickness of cover-glass (Fig. 9) . 



6. A one-twelfth-inch oil-immersion achromatic, and either 

 a 3-mm. or 2-mm. (N.A. 1*30 or 1*40) apochromat. The 3-mm. 

 apochromat of Zeiss should be 



chosen in preference to a 2-mm. 

 for general work. Its greater 

 focal length gives it a longer 

 working distance, and it may be 

 used with the highest 'power 

 oculars. 



The possessor of either series 

 of these lenses would have a 

 complete battery, and would be 

 in a position to deal with any 



Class 4of photo - micrographic FlG , 9 .-Apochromatic Objective 

 work. The apochromats Should fitted with Correction-collar, 



be chosen if possible ; but, as & - ( Zeiss -) 



already stated, it need not be 



regarded as an insuperable obstacle to achieving fine results if 



these cannot be obtained. (It should be noted that the 35-mm. 



and 70-mm. apochromatic objectives are made for direct 



projection or photography only, and should not be used in 



conjunction with an ocular.) 



In cases where such a complete outfit as the above cannot 

 be procured, then a one-inch and a one-sixth-inch lens will 

 probably be most useful for general work. If bacteriological 

 work is to be attempted, but little can be done without the 

 addition of a one-twelfth-inch oil-immersion objective ; and 

 where it is intended only to do the latter class of work, 

 then an oil-immersion objective, with a two-thirds-inch in 

 addition, could be made to answer all ordinary requirements. 

 It would certainly be better, if possible, to acquire, say, the 

 16-mm. and the 3-mm. apochromats, as with these two lenses, 



