654: PROCEEDINGS OF THE 



resemblance which the light dorsal marking bore to the Greek 

 letter tau. The generic name was altered to Lebertia (after 

 Leberts) by Neuman, in 1880. Sig Thor has now divided the 

 genus into five sub-genera. These are as follows : 1, Lebertia; 

 2, Pilolebertia ; 3, Mixolebertia ; 4, Pseudolebertia ; 5, Hexa- 

 lebertia. Three of the type species have been found in the 

 British area. The type species L. tau-insignitus at present has 

 only been found in the Lake of Geneva. But, being the type, we 

 have thought it right to include a description for comparison. 

 We have about ten species for the British area. One species, 

 Lebertia trisetica (Sig Thor), sub-genus Hexalebertia, was found 

 in Surrey in 1896. It was sent to Sig Thor, who named it in 

 1900 ; two specimens were taken at the time, but it has never 

 been recorded since. Several of the ten species have already 

 been recorded, but never fully described in Britain. * 



Mr. J. W. Gordon exhibited and described an objective of the 

 same type as that described by Mr. E. M. Nelson in the last 

 number of the Journal of the Club, as being the latest produc- 

 tion of Messrs. Zeiss. Mr. Gordon explained that his lens a 

 1/2-in. dry lens, fitted with a front carrying an oil-immersion lens, 

 had been constructed for him by Messrs. Beck so far back as 

 July 1909, that he has had it in use ever since, had shown it to 

 various persons, and that the lens had been described in the 

 catalogue of the Optical Convention of 1912. The following is 

 the description which appeared there : ' The use of oil-immer- 

 sion has hitherto been confined to objectives of the 1 /8th- in. and 

 l/12th-in. class under an impression, which proves to be mistaken, 

 that oil-immersion secures no particular advantages when 

 applied to objectives of lower power. The model is a 1/2- in. dry 

 lens fitted with a supplementary lens of rather less than hemi- 

 spherical angle, mounted so that the centre of the sphere lies 

 in the object. The spherical surface, therefore, produces no 

 refraction, and its addition to the optical system involves no 

 change in the correction of an objective adjusted for viewing 

 an uncovered object. The abolition of the top surface of the 

 cover-glass, by oiling on the supplementary front lens produces 

 an increase of 50 per cent, in magnifying power, and a com- 

 mensurate increase in light-gathering power. The catoptric 

 haze produced by internal reflection from the front face of the 

 permanent front lens sinks into comparative insignificance, and 



