An Improved Method of Apertometry. 347 



greater certainty, when the eye examines the apertometer itself 

 in place of its greatly-diminished injage. Owing however to the 

 larger surface the light source has to be brilliant, particularly if 

 outside light has to be competed with. I have found a pocket 

 electric-battery torch admirable for the purpose. For objectives of 

 medium and high power the bull's-eye lens frequently fitted to 

 such lamps may with advantage be retained. 



For dry high-power lenses and for oil and water immersion 

 systems the use of a plate similar to the Abbe instrument is neces- 

 sary. The use of the correct tube length is not of such importance 

 as it is with objectives of longer focal length. If the descending 

 light path be employed both the indices and auxiliary objective 

 may, however, be dispensed with by having the curved edge of the 

 plate frosted and graduated. By this means the boundary of the 

 cone of light may be observed and measured, without the necessity 

 of indices or hand magnifier (Eamsden ocular). Although these 

 simplified methods do not give the same degree of accuracy as that 

 which the more complicated method provides, yet in my hands they 

 appear quite equal to the original Abbe method, and an accuracy 

 to • 01 N.A. should be easily obtainable. A further advantage not 

 previously mentioned which the use of a descending light path 

 provides is that since the auxiliary objective is not employed there 

 is no possibility of the error arising which Spitta (7) has drawn 

 atttention to. 



For very low aperture objectives, Conrady's method would 

 appear to be very reliable, and to be accurate to a least 1 p.c, if 

 proper precautions have been taken with regard to tube length. 

 (The relatively long focal length of such objectives renders the use 

 of the correct tube length of the greatest importance.) In this case 

 also, however, in my opinion, there is definite advantage to be 

 obtained by the employment of the descending light path. 



Section 10. — Application of the Method to the Sub- 

 stage Condenser. 



For many purposes it is necessary to obtain the values of 

 apertures, iris-diaphragms, or central stops placed in the lower 

 focal plane of the condenser in terms of N.A. This may be done 

 in several ways ; probably the most accurate hitherto described 

 was that employed by the author for measuring working aper- 

 ture (9). Better results should be obtainable by means of a 

 direct method by employing the apertometer in the same way 

 as that described above for ttie objective. For this purpose the 

 Abbe apertometer is placed on the stage upside down, and the 

 rays from the condenser caused to enter the plate in a similar 

 manner. It will be at once observed, however, that the method 



