199 



magnification being therefore 18 '6 diameters. With the draw- 

 tube racked right out, 10 spaces of the stage micrometer are 

 observed to be equal to 28 spaces of the eyepiece micrometer, 

 equivalent to a magnification of 2S diameters. An increase, in 

 this case, of 73 mm. has therefore produced an increase in mag- 

 nification of 9 '4 diameters, and, on dividing the first number by 

 the second, according to Abbe's rule, the equivalent focus of the 

 objective is found to be 7 '8 mm. On multiplying this latter 

 figure by the magnification found in the second measurement, it 

 is found that the upper focal plane of this objective lies 217 mm. 

 below the eyepiece micrometer, and by applying a millimetre 

 scale it is found that the upper focal plane of this objective lies 

 14 mm. below the shoulder of its standard screw. The second 

 stand was arranged to show " why resolving power is dependent 

 on aperture." The Abbe Apertometer was next dealt with ; 

 several stands were arranged showing the normal use of the 

 apertometer, its limitations, and method of using with an 

 objective, having a large back lens. The differences between 

 ordinary and compensating eyepieces received due attention, and 

 were well shown on stands having an Abbe test-plate as an 

 object. The Abbe test-plate was also used in succeeding exhibits 

 as a test for spherical aberration, chromatic aberration, defects 

 in the centring of the lenses of an objective, and to show the 

 effect of varying cover-glass thickness. There was also exhibited 

 in an adjoining room the Watson-Conrady apparatus for photo- 

 micrography, in which the chief point to notice was that the 

 usual large and entirely uncorrected condenser is replaced by a 

 small but strictly achromatic and aplanatic condenser of 2|-in. 

 diameter and 3-in. focus, acting in conjunction with an auxiliary 

 iris. It was shown that this form is considerably more efficient 

 than that usually employed, and actually leads to gain in light 

 owing to the perfect transparency of the comparatively small and 

 thin condenser lens in contradistinction to the usual thick and 

 green common condensers. 



Mr. F. Chapman, A.L.S., F.R.M.S., communicated a paper on 

 " Recent Foraminifera of Victoria." 



Mr. A. Earland said that before proceeding to give an abstract 

 of Mr. Chapman's paper, which would be extremely valuable as 

 a work of reference, but which he feared would not prove very 

 interesting reading to members present, he had been asked to say 



