284 PKACTICAL PHOTO-MICKOGKAPHY 



does not accurately indicate its magnifying power, the camera- 

 length has not been mentioned. 



The objects are not placed in the absolute order of their 

 difficulty. Some sacrifice has had to be made in this respect 

 to enable them to be reproduced within the limits of the space 

 at disposal. The prints are from untouched negatives. 



PLATE I, FIG. 1. Transverse Section of Stem of Clematis. 

 X 20. 



Double stained with carmine and iodine-green. 



Objective, 35 mm., Zeiss apochromat (for direct projection 

 without ocular). 



Camera-extension, 2 ft. 9 in. 



Illuminant, 1 -ampere Nernst electric lamp. 



No sub-stage condenser. 



Convergent beam from Nelson's achromatic bull's-eye, 

 projected so that the image of the radiant was focussed approxi- 

 mately at the surface of the front lens of the objective. 



Screen, saturated solution of picric acid. 



Exposure, 5 seconds. 



Plate, Imperial orthochromatic (extra rapid). 



Development by metol-hydrokinone. 



PLATE I, FIG. 2. Transverse Section of Stem of Beech. 



X 20. (Double stained, as above.) 



Objective, planar 42 mm., by James Swift & Son. 



All other conditions as in Fig. 1. 



Figs. 1 and 2 are photographed under exactly similar 

 conditions to show the difference that exists between ordinary 

 low-power objectives even those of a high class, such as 

 the apochromatic lenses and the later types of low powers, 

 such as planars and micro -summars. It will be seen on 

 carefully looking at Fig. 1 that the definition falls off de- 

 cidedly near the edge of the specimen although the central 

 portion is perfect, whereas in Fig. 2 the definition at the edge 

 is as good as at the centre. Both these photographs are but 

 portions of the original negative, approximately only a quarter 

 of the available field being shown in the reproduction. In 

 photographing such preparations therefore it is necessary to use 

 a lens which has a large field rather than one that possesses 

 corrections of an extremely high order confined to a smaller field. 



