ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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the prism-table. The collimator is supported by a very strong pillar 

 situated on one of the legs of the tripod. The slit opens symmetrically ; 

 its cheeks are of hard steel ; it is first accurately adjusted for infinity 



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and then firmly secured to the collimator. A drum interval of the 

 micrometer-screw corresponds to O'Ol mm. A rotatory disc s is in front 

 of the slit, and is useful in connexion with the camera, It can be easily 

 exchanged for two discs for photometric measurements in the case of very 



