QUEKETT MICROSCOPICAL CLUB. 659 



back lens of the objective ; this minimises the unavoidable loss 

 of working distance. The additional lens may very conveniently 

 be fitted to the " funnel stop ' commonly supplied with oil- 

 immersions to reduce the aperture for dark-ground illumination. 

 With an oil-immersion thus converted to a water-immersion, it 

 is useless to expect that the whole aperture will be available ; 

 the corrections of the objective are far too much upset for that ; 

 but if the additional lens is made of such diameter as to 

 reduce the N.A. to about 1*1, and if an illuminating cone not 

 exceeding N.A. 0'75 or 0*8 be employed, the performance is 

 in all the cases tried quite up to the standard of the ordinary 

 water-immersion and better than some. It should not be 

 forgotten that, the substitution of water for oil renders the 

 objective sensitive to changes of cover-thickness, and the tube- 

 length will have to be carefully adjusted to compensate for this. 

 It is hoped that this method of converting an oil-immersion 

 into a water-immersion may be found of use, especially to 

 those who occasionally require to use a water-immersion for 

 work on living specimens, or in other work for which an oil- 

 immersion would be inconvenient. 



Objects were exhibited under microscopes kindly lent by 

 Messrs. H. F. Angus & Co., and by Messrs. W. Watson & 

 Son, to illustrate the paper ; these were : 



1. A specimen of polished steel, with a Watson 4 mm. apo- 

 chromatic, N.A. 0'85, a convex lens of 6 diopters being used 

 to correct for the absence of a cover-glass. (Magnifying power, 

 340 diameters.) 



2. Bacillus typhosus, showing flagella, with a Watson l/6th, 

 N.A. 0'74, an extra cover being introduced to bring the total 

 thickness up to 0'50 mm. and a concave lens of 8 diopters being 

 introduced. (510 diameters.) 



3. Tubercle bacillus, with a Watson l/12th oil-immersion, 

 N.A. 1*3 working with a water-immersion, a convex lens of 

 -j-10 diopters being introduced to effect the conversion. (940 

 diameters.) 



The Hon. Secretary (Mr. F. Burton) then read ' ; Notes on a 

 Diatom Structure;' by Mr. A. A. C. Eliot Merlin, F.R.M.S. 

 The author drew attention to a very beautiful form of tertiary 

 structure he recently found on a variety of Aulacodiscus comberi 

 from Oarnaru. The valve is on a styrax type slide of 230 forms 



