PROCEEDINGS OF THE SOCIETY. 539 



of ordinary make, and lie thought the way in which the structure was 

 resolved was quite satisfactory. 



Mr. Rheinbcro; thought that the use of a black stop over the objective 

 might, with a sufficiently strong source of light, prove more useful for 

 indicating the presence of structure beyond the resolving power of the 

 objective, on the lines of the Siedentopf methods, than for giving 

 satisfactory images of detail which the objective was competent to 

 resolve. Images obtained in this way were not very reliable, as objec- 

 tionable diffraction effects were introduced by stopping out the central 

 beam, and it was quite possible to get the structure duplicated. This 

 applied specially to structures anywhere near the limit of resolution of 

 the objective, when used in this manner. With anyalatum, curiously 

 eaiough, the dots were seen fairly well under the conditions in 

 question, owing to the peculiar distribution of the spectra ; but if other 

 diatoms were tried the results would be more at variance with the 

 correct image. He thought it would be better if, instead of stopping 

 out the central light wholly with a black spot, they stopped out certain 

 colours only ; they would get contrast and at the same time be free from 

 the objection mentioned. He had been led to examine the effects of 

 black and coloured stops over the objective some years ago when ex- 

 perimenting on colour illumination, and had at the time referred to them 

 in a paper read before the Society.* 



Mr. dirties said he quite agreed with what Mr. Rheinberg said, as 

 it was quite true that he could only resolve angidatum and formomm 

 in this way ; he was unable to do this with smaller diatoms he had tried, 

 except so far as the coarse structure was concerned. 



The thanks of the Meeting were voted to Mr. dirties for his 

 exhibit. 



Mr. Rheinberg called attention to an experiment in connection with 

 the theory of ■ -Microscope images, which he had fitted up in the room, 

 showing that the appearance of a grating could be produced in the Held 

 of the Microscope without there being anything on the stage. The lines 

 seen were achromatic interference bands, produced with the help of two 

 Thorp gratings of equal pitch placed behind the objective. A brief 

 resume of a paper on the subject recently read at the Optical Convention 

 was given. 



Mr. Rousselet called attention to an exhibit in the room of a living 

 specimen of PlumateUa punctata Hancock, sent by Mr. John Hood, of 

 Dundee. This seemed to be a very rare fresh-water Polygon, which 

 has not apparently been recorded in England since its first discovery by 

 Hancock in 1850. It differs from PI. repens and its varieties mainly by 

 having a soft, gelatinous and transparent ectocyst, which spreads in 

 rather stout branches horizontally on stones. 



Mr. E. M. Nelson's paper on " The Tubercle Bacillus " was taken as 

 read. 



* " Notes on Colour Illumination, with special ret', rence to the choice of suitable 

 colours," J.R.M.S., 1899, pp. 143-4. 



