364 



Mr. Karop exhibited and described a little stop which had 

 been brought to the meeting by Mr. Swift, and made to fit the 

 diaphragm carrier of an Abbe condenser, for affixing coloured 

 gelatine in colour-ground illumination. 



Mr. W. Stokes read a note on "The Cause of Multiple Images 

 from Mirrors," the subject being illustrated by diagrams drawn 

 upon the board. 



Mr. E. M. Nelson thought they were to be congratulated on 

 having this matter brought before a meeting of the Club, for so 

 far as he was aware it had never been satisfactorily cleared up 

 before. He thought the author of this paper had succeeded in 

 showing the cause of all the trouble. 



Mr. Hardy inquired if a mirror silvered on the upper surface 

 in the same way as the mirror of a reflecting telescope had ever 

 been tried ? 



Mr. Ingpen said this had been done, but it had been found 

 impossible to keep the surface good for any length of time. 



Mr. Michael remarked that many persons used a prism which 

 came to the same thing. 



Mr. Stokes said he could see no great advantage in having 

 the surfaces parallel. 



Mr. Rousselet thought it was to be inferred from the paper 

 that to get rid of the defect it was necessary that the two sur- 

 faces should not be parallel. 



Mr. Stokes said they certainly got the multiple images when 

 they were parallel. 



Mr. Nelson said he had rather a curious form in an old 

 microscope, a concave mirror with an exceedingly long focus; 

 so long, indeed, that it did not practically affect the image ; this 

 did not show a multiple image. 



Mr. Michael thought there was nothing equal to a prism for 

 the purpose. 



Mr. Ingpen said these mirrors used to be made at one time 

 with a very slight curve, because they could not make them 

 truly flat. 



The thanks of the meeting were voted to Mr. Stokes for his 

 paper. 



Mr. Rheinbcrg's paper " On a New Differential Coloured Sub- 

 stage Illuminator " was read by Dr. Measures, and the 



