QUEKETT MICROSCOPICAL CLUB. 269 



A letter from Mr. T. Midgley, of Bolton, was communicated to 

 the Club by Mr. Carter, recommending the use of " Necoloidine " 

 as a substitute for celloidin. Mr. Midgley had used this substance 

 as an embedding medium for cutting sections of cotton and other 

 fibres with complete success. 



The Secretary announced that a quantity of deep-sea deposits 

 had been received from Mr. Ham for distribution. He proposed 

 to divide them up into packets and to distribute them to members 

 interested in due course. 



The President drew attention to the fact that the unwritten 

 law that smoking should not be indulged in at the meetings 

 had occasionally been violated, and that the committee's attention 

 had been drawn to the matter. The committee had decided that 

 the members should be asked to refrain from smoking at any 

 of the meetings. 



Mr. Merlin was then called upon to read his paper on 

 " Black- and White- Dot Focus." Mr. Merlin thought it might 

 be of interest if the minute secondary structure of Pleuro- 

 sigma formosum which Mr. Brown had mentioned at the 

 previous meeting as having been shown to him by Mr. Nelson, 

 and with which he had long been familiar, could be photo- 

 graphed. He made the attempt, and submitted the resulting 

 photograph to the meeting. The magnification was 4,000 

 diameters, the objective used an apo. 1/12 in., N.A. 1*4, 

 illuminating cone N.A. 1-3, illuminant sunlight. The main 

 structure was represented in dark- dot focus, the minute secondary 

 perforations, usually about 4, being indicated by darker shading 

 near the margins of the primaries. The diatom was mounted 

 in styrax. The secondaries always appear black at a definite 

 focus, and no " white-dot " effect can be obtained. Mr. Merlin 

 considers them to be perforations in the capped structure of the 

 primaries. Diffraction effects were reduced to a minimum 

 owing to the very large illuminating cone employed. Mr. Merlin 

 also showed two photographs of the secondary structure of 

 Triceratium sp. showing the "white-dot" and "black-dot" 

 effects. In the latter the framework of the diatom was sharp, 

 while in the former it was out of focus. Mr. Merlin finds the 

 black dots to be sharp and at the level of the convex upper 

 surface of the valve framework, while the white dots are merely 

 optical images formed by the structure at a level well above the 



