370 



The Chairman said it appeared to him to be rather a novel and 

 probably convenient addition, as the twirling of the small spindle 

 provided a kind of intermediate adjustment between the ordinary 

 coarse and fine adjustments. 



Mr. Swift said he had always understood that the small milled 

 top of the conical fine adjustment head in Continental micro- 

 scopes was for the purpose of obtaining a rapid motion, exactly 

 in the same way as the two-speed adjustment now exhibited. 



The thanks of the meeting were voted to Messrs. Watson for 

 their exhibits. 



Mr. Rheinberg read a further paper on "The Black and White 

 Dot Phenomenon," which he regarded as due to the differences of 

 refractive index of the transparent diatom silex and of the 

 medium in which the object was mounted. The argument was 

 supported by reference to diagrams on the board showing the 

 ratio between the angles of total reflection of difl^erent mounting 

 media. 



Mr. Stokes said he regarded this as a very interesting subject, 

 and could fully appreciate the amount of work which Mr. 

 Rheinberg had devoted to it ; but he was hardly able to follow 

 some of the conclusions given in the paper. He was not able to 

 imagine that a diatom was perforated wuth holes having a depth 

 greater than the breadth. With regard to diffraction, it was 

 useless to deny that this had an overwhelming part in micro- 

 scopical vision, 01' that the diffraction effects which were noted in 

 coarse objects were different from those in the case of fine objects. 

 He thought that diffraction of aperture, which was a telescope 

 theory, was something of the same kind, and that a star disc 

 showed very similar effects, for both had rings w^hich to some 

 extent limited the disc and formed a limit of resolution, and the 

 appearance of the white dot of a very fine perforation was very 

 much like the appearance of a star disc. As regarded the mark- 

 ings of Pleurosiyma angulatum, his work convinced him that 

 these diatoms were altogether peculiar, because they seemed to 

 act in a manner quite different from all others. For this reason 

 a great deal of work had been given to them, and they had given 

 rise to more illuminators than anything else. He should be very 

 pleased to read this paper wh'in it appeared in the Journal. 



Mr. Rheinberg said that he thought Mr. Stokes was mistaken 

 as to the depth and breadth of the holes in diatoms, and that he 



