193 



PROCEEDINGS 



OP THE 



QUEKETT MICROSCOPICAL CLUB. 



At the meeting of the Club held on March 15th, 1907, 

 Dr. E. J. Spitta, F.R.A.S., F.R.M.S., President, in the Chair, 

 the minutes of the meeting held on February 15th were read 

 and confirmed. 



Messrs. J. A. Robertson, William Browett, and Cecil Worssam 

 were balloted for and duly elected members of the Club. 



Mr. Conrad Beck, F.R.M.S., gave a demonstration on u The 

 Illumination of Opaque and Unmounted Objects." A large 

 number of microscopes were arranged to illustrate the points 

 dealt with and the apparatus usually employed. It was stated 

 that the principle of all opaque illumination is to throw upon the 

 object a small and very brilliant image of the source of light. 

 With low powers, say from 3 in. to h in., the light can be thrown 

 upon the object by means of either the bull's-eye condenser or 

 the side condenser. The following points in manipulation of the 

 bull's-eye condenser should be observed : — (1) The light should be 

 as near as is convenient, say about 10 in. (2) The light, bull's- 

 eye, and object should be in line. (3) The light should be 

 above the level of the stage as high as the object-glass will admit, 

 so that the shadows shall not be exaggerated. (4) The bull's-eye 

 should be so placed that a small image of the light source is 

 focussed on the object. Character of illumination — one-sided. A 

 second method of using the bull's-eye enables opaque objects to 

 be examined with powers such as J in. or | in. The following 

 points to be observed : The light to be slightly below the level of 

 the stage. Bull's-eye to be placed, flat surface upwards, almost 

 touching the stage. The edge of the lamp-flame should face the 

 stage. A beam of light is reflected from the flat surface, and 

 condensed by the curved surfaces into a flat feather in an almost 

 horizontal direction. The shadow will be very marked owing to 

 the great obliquity of the light, and for this reason surfaces to 

 be examined must be almost plane. For low powers a more 



