386 PROCEEDINGS OF THE 



compared with the human retina, the lecturer said a photograph 

 might be taken of a quickly moving vehicle, such as a dog-cart, 

 when on the plate the spokes of the wheels would all be distinctly 

 evident and well defined, but to an observer's eye they would 

 present nothing but a blurred and indistinct view. The use of the 

 camera lucida for securing representations of the magnified image 

 and some of the difficulties of its employment were then referred 

 to. It was remarked that in the hands of such an expert as 

 TufEen West the results were excellent, but the personal factor 

 •comes in strongly in the use of the instrument, and not infre- 

 'quently the resultant drawing is a representation of what the 

 observer thinks he sees, or has seen represented elsewhere, rather 

 than of what is actually present. The chief objection usually 

 found in the photograph is the lack of depth of focus, but this 

 can be supplied to a great extent by the use of a Davis's shutter 

 behind the objective, cutting down the aperture of the objective ; 

 in fact, dealing with the micro-objective in the way that a 

 photographer would stop down his lens, say, to f/32. The 

 rule of the use of a small aperture for securing crisp definition 

 and obtaining sharpness in different planes is well recognised by 

 the camera expert, and Dr. Rodman attributed a great deal of 

 his success in photomicrography to his observance of that rule. 

 Various methods of illumination for photographic purposes were 

 discussed, the Nernst lamp, when obtainable, being recommended. 

 As an example of the results obtained by the photogi'aphic and 

 the camera-lucida methods, the lantern view of a beautifully 

 finished pen-and-ink dra\\dng of Paramecium in conjugation, at a 

 magnification of x 300, was thrown on the screen. In this the 

 details of structure came out very finely, the cilia round the 

 bodies being perfectly distinct. The photographic record in 

 this case was unsatisfactory, details being absent. But it was 

 confessed that the camera-lucida drawing was not exactly the 

 true record as obtained with that instrument : it had been 

 improved afterwards and details added. The photomicrogi'aph 

 obtained from the same micro-slide from which the camera- 

 lucida drawing had been made was then projected for com- 

 parison, and showed by an absence of many of the characters 

 of the drawing that the latter had been produced at the cost 

 of a good deal of artistic licence, and was quite untruthful in 

 many points. In another example, however, the photomicro- 



