APPENDICES 809 



' To obtain a suitable illumination for the use of the aperto- 

 meter the light should be made to fall in a horizontal direction 

 upon the cylindrical sides of the glass plate. In the event 

 of the source of light not being wide enough to send sufficient 

 light to both pointers, so as to cause them to appear brightly 

 illuminated at the same time, the difficulty may be overcome 

 by placing in front of the microscope a semi-circular translucent 

 screen, e.g. an appropriately bent sheet of thin white paper. 

 The upper edge of this screen should extend a little above the 

 top of the apertometer. 



' The power of the eye-piece used on the auxiliary microscope 

 should rise with the magnifying power of the objective under 

 examination, so as to obtain a sufficiently large image of the 



FIG. 79. 



pointers, &c., but on no account should an eye-piece be used 

 the diaphragm of which partly cuts down the bright circle 

 of light as seen through the auxiliary microscope. 



' To measure with the apertometer low apertures (less than 

 0*4 or 50), which are generally associated with long foci of 

 7 mm. and more, the pointer, &c. should be viewed with the 

 unaided eye, without the auxiliary microscope. To this end 

 in view the silver disc should be focussed, as described, and 

 the eye-piece removed and replaced by a disc of cardboard 

 or metal with a central hole. By this means the eye is kept 

 in a central position whilst looking towards the objective.' l 



The apparatus is quite simple in use, and presents no unusual 

 difficulty to a microscopist of even limited experience. 



1 See Directions for Use of Abbe Apertometer, by Carl Zeiss. 



