198 



that there was here a good opportunity for work with the highest 

 powers of the microscope, and that it seemed to him a great 

 pity that so many good achromatic and apochromatic objectives 

 should be employed simply to resolve diatom structures which had 

 been resolved a thousand times before ; while such practice was 

 good training for the expert manipulation of the instruments, it 

 should be considered simply as a preliminary to higher things. 



Mr. Scourfield then read a paper on " The Collection and 

 Preservation of Fresh-water Rhizopods," communicated by our 

 honorary member Dr. Eugene Penard, of Geneva. 



Considerable discussion followed, in which Messrs. Earland, 

 Bryce, Karop, and Stokes took part. 



At the meeting of the Club held on May 17th, 1907, Dr. E. J. 

 Spitta, President, in the Chair, the minutes of the meeting held 

 on April 19th were read and confirmed. 



Messrs. J. Drinkwater, B. Drinkwater, C. E. Graham, J. W. 

 Ogilvy, and Captain C. Ackerman were balloted for and duly 

 elected members of the Club. 



Mr. F. W. Watson Baker, F.R.M.S., gave a demonstration 

 dealing with " Some Methods of Ascertaining the Qualities, 

 Defects, and Characteristics of Micro-Objectives." The first 

 point treated was the determination of the equivalent focus of 

 objectives by Abbe's method. This method gives the true 

 equivalent focus of an objective as well as the position of its 

 upper focal plane, and is obtained by two measurements of a 

 stage micrometer with different extensions of draw-tube. The 

 principle of the method is that there is no magnification at all 

 in the upper focal plane itself, and that the magnification 

 increases uniformly by one diameter, for every increase of the 

 distance between the upper focal plane and the projected image 

 by an amount equal to the equivalent focus. The method was 

 practically shown in the first microscope then exhibited. A 

 stage micrometer divided to one-hundredth of a millimetre was 

 the object employed, and an eyepiece micrometer divided to 

 one-tenth of a millimetre used in a Iluyghenian eyepiece, the 

 field-lens of which had been temporarily removed, as is abso- 

 lutely necessary for this purpose. When the draw-tube was 

 racked right in, it was observed that the spaces of the stage 

 micrometer covered 18 '6 spaces of the eyepiece micrometer, the 



