99 



C. Apertometer, Test-plate, Diffraction Apparatus. 



1 ABBE's Apertometer. 



A simple apparatus for determining the numerical and angular aperture of 

 objectives - the Apertometer - - was designed by Professor ABBE in 1876. 

 It is adapted for any large or medium stand fitted with a draw -tube. A low- 

 power objective of about 40 mm focal length, which screws into the lower 

 extremity of the draw -tube, is supplied with the instrument, but under appro- 

 priate stopping the BERTRAND lens may, with Mineralogical Stand l b , be used 

 instead. The auxiliary microscope, formed by the combination of the low-power 

 objective with the eye -piece, being focused upon the emerging pupil of the 

 objective to be examined, the numerical aperture of the latter may be read off 

 the apertometer by means of two movable metal indicators. 



Further particulars regarding the principles of construction of the apparatus 

 and the method of measuring with it are contained in the "Journ. of the R. Micr. 

 Soc." (1), vol. I, 1878, pp.19 22 and (1), vol.111, 1880, pp.20 31; see also DIPPEL, 

 "Das Mikroskop", vol. I, pp. 348 352, and the directions accompanying the 

 instrument. 



We supply two different patterns of the apertometer; in the form shown 

 in Fig. 56 the indicators move upon the glass disk direct, whereas in the second 



C.Zeiss . Abbe ' s Jena 

 Apertometer. 



View from above. 



Fig. 56. 

 Apertometer, No. 155. 



Side view. 



