THE OPTICAL EQUIPMENT 



71 



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this apparatus (Fig. 20), it will be seen that the image of the 

 radiant produced by this condenser A is focussed in the plane 

 of an iris- diaphragm C, by means of which the beam can be 

 reduced to such a diameter as just suffices 

 to cover the object which is being photo- fo 



graphed, and consequently any unnecessary 

 light is kept out of the field of the micro- 

 scope. Another iris-diaphragm B is placed 

 close to the condenser itself, and by closing 

 this the width of the beam of light can be 

 adjusted so that the sub-stage iris of the 

 microscope is filled with light as required ; 

 by the combined use of these two iris-dia- 

 phragms the light passing through the 

 condensers and reaching the microscope is 

 therefore kept down to that actually in use 

 when taking the photograph. An auxiliary 

 single lens is placed close to the iris-diaphragm 

 on which the image of the radiant is received, 

 and this lens is of such focus that it pro- 

 duces a fairly sharp image of the large 

 aplanatic condenser on the iris of the sub- 

 stage condenser D of the microscope. It 

 therefore serves a similar purpose to that 

 of the field-lens of an ordinary eye-piece, 

 directing and converging the beam on to the 

 sub-stage condenser. This apparatus has 

 been found to fulfil all that is claimed for 

 it, and where it is convenient to have the 

 radiant at some distance from the microscope 

 it is very useful. Some photographs taken by 

 this arrangement under critical conditions 

 will be seen among the illustrations of this 

 book. 



A complete collecting-lens system that 

 may be used with objectives of any power is 

 that made by Messrs. Zeiss to the computation of Dr. Kohler. 

 In this arrangement three lenses" are used, and as these are 

 supplied by Messrs. Zeiss they are j distinguished by the 

 Eoman figures I, II, and III. By arranging these in different 







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