QUEKETT MICROSCOPICAL CLUB. 275 



Microscopists were recommended to close down the condenser 

 diaphragm as far as possible and even to cut out the "useless central 

 dioptric beam." Under those circumstances it is hardly to be 

 wondered at that it was stated that with the means employed 

 no definite inference could be drawn of the real structure of the 

 Abbe diffraction plate or of Pleurosigma angulatum. Mr. Merlin 

 here quoted the diffraction experiments described by Stephenson 

 in the Monthly Microsco'pical Journal, Vol. XVII, pp. 88 and 87, 

 and he submitted that had Abbe's inferences from his diffraction 

 theory been true, the microscope would have been the most un- 

 reliable instrument imaginable. Mr. Merlin was unable to obtain the 

 false effects described, the only result of reducing the aperture 

 being to increase the size of the antipoint and consequently to 

 reduce resolving or separating power. It was at the Q.M.G. 

 that Mr. Nelson read the papers which caused Abbe to modify 

 his ideas in many respects and opened an easy road for the student 

 who is anxious to employ his lenses to the best advantage. Mr. 

 Merlin recommended the study of Mr. Nelson's paper on " The 

 Substage Condenser " read before the Club on May 16th, 1890. 

 He said that if the principles there laid down had been followed 

 by working microscopists in general, enormous strides would have 

 been made in the elucidation of many difficult structures. " Criti- 

 cal illumination," as described by Nelson, consists of focusing 

 the image of a suitable radiant on the object and employing a 

 large working aperture of the objective. The radiant used by 

 Nelson is the edge of a half-inch paraffin lamp flame. Mr. Merlin 

 said that the objections that had been raised to the use of the 

 flame edge on the grounds of its thickness were probably due 

 to the use of a condenser unprovided with an arrangement for 

 making adjustments for slip thickness. It is often impossible 

 to fiU the back lens of the objective evenly without this adjust- 

 ment, which is very rarely fitted to condensers, and for which 

 opticians say there is no demand. The three necessary conditions 

 for good visual work are (1) a large working aperture, (2) a well- 

 defined image of the radiant focused on the object plane, and (3) 

 an unbroken even disk of light at the back of the objective. The 

 lamp flame should be at a distance of about 8 inches. The dry 

 condenser used by Mx. Merlin is the Powell 4/10 apo. of 0-95 N.A. 

 with lever adjustment for slip thickness. It was described by 

 Mr. Nelson in the Journal R.M.S. for 1895, p. 231. To obtain the 

 JouRN. Q. M. C, Seeies II.— No. 87. 19 



