1897.J MICROSCOPICAL JOURNAL. 3 



A Simple Means of Comparing the Apertures of Objectives. 



15 V R. B. L. RAWIvINGS, 

 JNASHVlLIvK, TENN. 



While the subject of aperture is of interest to every 

 worker who [trizos liis objectives and wishes to under- 

 stand their exact capacity, the liigh price of the Abbe 

 Apertometer leaves the great majority of microscopists 

 without a means of determining aperture. 



From ntimerous tables which have been published com- 

 paring the actual ajjerture of lenses with what is claimed 

 for them, it is seen that in many instances the perform- 

 ance of the objective cannot be what is claimed for it. 



A search amongst the catalogues at hand of several of 

 the leading opticians of the world fails to show an aper- 

 tometer of any description listed in any of them, with 

 the single exception of the Abbe, listed by Zeiss. 



While the idea of the arrangement in the experiment 

 below detailed is suggested from a study of the Abbe 

 form of apertometer, it is essentially different in half the 

 technique. 



For the benefit of those who are not familiar with the 

 instrument and in the hope that I may make the proposed 

 modification plainer, it may not be amiss to attempt a 

 short explanation of its working, particularly as this is 

 not done in the Zeiss catalogue. 



It consists essentially of (a) an auxiliary objective and 

 (b) the plate glass semicircular and prismatic disc. 



The objective has a focal distance of about 8 inches, is 

 mounted with a society screw and has screwed in the 

 upper j)art of the mounting a cylinder with a small 

 diaphram in its upper end. 



This objective is to be screwed into the lower end of 

 the draw tube after the objective to be examined has been 

 focussed on the disc, care being used not to disturb the 

 focal arrangement of the objective in the nose piece. Its 

 purpose is for the reading of the indices. The draw tub*' 



