390 A. A. MERLIN ON BACILLI OF BUBONIC PLAGUE. 



endoplasm of this nature is very commonly existent in the longer 

 segments. 



Although the figures are drawn on a larger scale, a power 

 of about 2300 diameters was found advantageous, and was 

 generally adhered to in these observations, this magnification 

 being obtained by means of an apochromatic gth of N.A l'4r used 

 in conjunction with a 27 ocular. A solid axial cone of about 

 1*2 N.A. from an oil- immersion condenser was found to yield the 

 best results ; but the full aperture of Powell's dry apochromatic 

 condenser also proved satisfactory, exact adjustment for thickness 

 of slip being in the latter case eftect*^d by the correction collar. 



Strictly critical illumination is of course a sine qua non when 

 dealing with such minute structure, and Giflford's blue-green glass 

 and glycerine methyl-green screen should be employed. Under 

 these conditions I have seen most of the points alluded to with a 

 good objective of N.A. 1"3, but poor lenses are not likely to prove 

 adequate. 



In this and other delicate work under the highest powers, so 

 far as my experience extends, it would appear to be almost as 

 necessary that the substage condenser should be in as exact 

 adjustment as the objective itself. With regard to this matter, 

 it would doubtless prove a boon to many workers were the 

 thickness of slip for preparations requiring any considerable 

 magnifying power to be standardised, so that the many excellent 

 dry condensers of large aplanatic aperture now obtainable in 

 fixed mounts could be relied upon to work well on the majority 

 of mounted slides, which is far from being the case at present. 

 Well corrected oil-immersion condensers are practically exempt 

 from this drawback, as they perform satisfactoiily through slips 

 of very varying thickness ; but their constant employment, when 

 a cone exceeding N.A. 1-0 is not required, involves much useless 

 trouble and loss of time. 



I trust that I shall be pardoned by careful workers for having 

 thus dwelt on matters that are familiar to them, but in so doing 

 I have sought to emphasise the fact that the rough-and-ready 

 manipulation of an instrument furnished with an unsuitable con- 

 denser cannot be expected to reveal the structural details referred 

 to in this note. 



Journ. Quekett Microscopical Club, Ser. 2., Vol. VII., No. 47, November 1900. 



