SOME PROGBESSIVE EXAMPLES 285 



PLATE I, FIG. 3. Proboscis of Blow-fly, x 100. 



One-inch achromatic objective, N.A., 0-30, by James Swift 

 & Son ; mediar ocular, 3. 



Sub-stage condenser, Swift achromatic with top combina- 

 tion removed. (Working N.A., approximately 0-28.) 



Auxiliary condensers, Zeiss Nos. I and III. 



Illuminant, mercury- vapour lamp. 



Screen, Gifford F-line. 



Camera-extension, 2 ft. 6 in. 



Plate, Wellington orthochromatic process. 



Exposure, 1J minutes. 



Development by pyro-soda, with half the normal quantity 

 of pyro in solution, to reduce as far as possible contrast in the 

 dense part of the preparation. 



This object is always regarded as a particularly good test 

 with low-power objectives. The illumination should be effected 

 with a cone of light as large as the objective will allow, and 

 all corrections must be most carefully made to ensure that the 

 fine spirals and the outlying hairs on the tongue are brought 

 out as perfectly as possible. The hairs themselves should, 

 where they are in focus, die away to a perfectly sharp well- 

 defined point. It is necessary to select a tongue that is mounted 

 as flat as possible, and that is not too dense in its densest parts ; 

 otherwise it is impossible to obtain detail in these parts of the 

 object without lessening that to be seen in the thinner parts. 

 A well-corrected objective will not of necessity require any 

 colour-screen ; but the object is usually such that the use of one 

 is beneficial in helping to bring out detail. Its colour is usually 

 so non-actinic that it may almost be regarded as an object 

 in black-and-white, presenting no colour difficulty, when 

 orthochromatic plates are used. 



PLATE II, FIG. 1. Group of Foraminifera. x 25. 



Opaque object. 



Objective, 35-mm. apochromat for direct projection, by 

 Carl Zeiss. 



No ocular. 



Camera-extension, 2 ft. 9 in. 



Illuminant, 4-ampere electric arc lamp, by Leitz. 



The object is mounted as an opaque one, and illumination 

 therefore has to be effected by obliquely incident light. The 



