108 PROCEEDINGS OF THE SOCIETY. 



Optical Company, iu wbich the cover correction was obtained by 

 varying the thickness of a film of glycerine placed between the front 

 lens and an external flat disk of glass, as described on p. 251 of the 

 same volume. It was similar in construction to one made by Mr. 

 Gundlach, and exhibited at a scientific evening in 1876, but showed 

 considerable imjirovement. The figure and colour were excellently 

 corrected, but the definition could hardly be considered brilliant. 

 The cover correction was remarkably quick and satisfactory, a small 

 alteration making all the difference between the best definition and 

 none at all. The apparent angle of aperture was very large, but he 

 thought part of it was " spurious," probably owing to reflection from 

 the edge of the glycerine film. The working distance was inconve- 

 niently small, and as the adjustment was made inside the objective, it 

 was close for very thin as well as for thicker covers. The method of 

 correction was a very interesting one, and one which he thought 

 might hereafter form a new point of departure in the construction of 

 objectives. 



Mr. Beck said that the Fellows would remember that recently a 

 question was raised by the American Microscopical Congress as to 

 whether the aliquot parts of an inch or of a metre should be used as a 

 universal standard of microscopical measurement, anl he then ven- 

 tured to suggest that they should give the matter their attention. He 

 had considered it, and he certainly should recommend that the 

 divisions of the metre should be adopted. It was asked at the time 

 whether the scales of the divisions of the millimetre could be obtained 

 in the event of their being required ; and having turned his attention 

 to the matter, he had arranged a micrometer in which both scales could 

 be seen. Having ruled a fiducial line, they had ruled on one side of 

 it yJo and yoVo of an inch, and on the otlier side the yi^ of a milli- 

 metre, so that having the two scales on one slide there would be no 

 longer any necessity for changing the slides every time they wanted to 

 make a comparison. There would be a further advantage in having 

 the two scales in this way for comparisons, because if there should 

 happen to be any error or inq:)erfection in the instrument used for 

 ruling, it would he common to both scales. He had much pleasure 

 in i^resenting one of these scales to the Society, and if any Fellow 

 found anything which could be improved he should be happy to 

 adopt the suggestion. 



A Discussion took place between Mr. J. Mayall, jun., and Dr. 

 Edmunds, as to the immersion prism referred to by the latter at the 

 October meeting (vol. i. p. 309), and which Mr. Mayall claimed to 

 have been originated and suggested by him, a claim which Dr. 

 Edmunds on the other hand disputed. 



The following were exhibited :— 



Dr. Hudson : — Seventeen coloured transparent drawings of rotifers. 



Mr. F. H. Ward : — (1) The micro-spectroscope and aj)paratus 



