PHOTOGRAPHIC INTRODUCTION 5 



an unequal background in the print is to be avoided. 

 The lime must be carefully moved from side to side 

 and up and down to secure this result. A con- 

 venient stand for the jet, which is capable of being 

 racked in both these directions with ease and 

 uniformity, was made at our direction by Mason 

 of Clapham. 



The condenser was always centred for each 

 objective, and critical light was used throughout. 



If dark spots appeared on the picture the lime 

 was turned on its axis, or the condenser just touched 

 so as to remove the marks without affecting the 

 definition. 



Occasionally we have employed a second con- 

 denser between the light and the substage con- 

 denser. It serves to broaden out the illuminant, 

 and is very useful to equalise the light over the 

 field. Nelson's quasi-achromatic condenser we have 

 found very useful for the purpose, fitted on a stand 

 similar to that employed for adjusting the limelight 

 previously described. 



The aperture of the iris must not be contracted 

 at all when photographing bacilli with high N.A., 

 for, if so, white defraction marks will appear round 

 each bacillus, and will spoil the final result. Only 

 when using low magnification for tissues must it be 



