75 



B. Appliances for measuring and counting 

 microscopic objects. 



fJeasuring apparatus. 



Ocular Micrometers and Stage Micrometers. 



The requirements of measuring microscopic objects are in most cases met 

 by the use of simple ocular micrometers, i. e., of small glass discs with divisions, 

 which are dropped into the oculars with the side bearing 'the division lines 

 downwards. The values of divisions are best determined by comparison with a good 

 stage-micrometer for each combination of objective and ocular. Approximate 

 values, which are in many cases sufficiently accurate, will be found in tables 

 supplied with each ocular micrometer. These tables apply to combinations of 

 achromatic objectives with HuYGENIAN oculars, and are calculated for the pre- 

 scribed (160 mm) tube-length. 



In combinations of the compensating oculars 6 with any of the apo- 

 chromatic objectives the approximate determination of the value of an interval 

 becomes very much simplified. The divisions in this ocular are so computed 

 that with a tube-length of 160 mm the value of one interval of the micrometer 

 scale represents exactly as many micra, /< (0.001 mm), as there are millimetres in 

 the focal length of the objective, i. e., Systems: 16 mm, 8 mm etc., value of inter- 

 val: 16 , 8 n etc. (See Zeitschr. f. wiss. Mikrosk., vol. V, 1888, pp. 150155.) 



As however these values also are still only approximate, a comparison 

 must be made with the stage-micrometer whenever more exact measurements 

 are required. 



Our regular manufacture of Ocular Micrometers is restricted to the 

 one form, catalogued as No. 117 : 5 mm divided in Vio mm. 



Other divisions to order. 



When ordering ocular micrometers please state whether they are required 

 for ordinary or for micrometer oculars. 



Ocular Micrometer, in case 



Marks Codeword 

 5. Paviloii 



