CHAPTER VIII 



CRITICAL PHOTOMICROGRAPHY 



In this chapter the methods of Critical illumination, as 

 defined by Mr. Nelson, and the modifications that have 

 been mentioned as suitable, are applied to photomicro- 

 graphy, and details are given of the adjustments and 

 processes adopted to take both low- and high -power 

 photomicrographs under those conditions. 



The experiments detailed in Chapter VI supply all the 

 data required for obtaining the best illumination possible, 

 critical light, and an even. field. The following rules are 

 based upon them, and should always be followed for 

 medium- and high-power objectives used either with or 

 without eyepieces. Low-power objectives are also best 

 treated in the same way provided suitable condensers 

 are available, but in most cases it is necessary with 

 objectives of 2" and lower power to make use of the 

 methods described in the last chapter to get an evenly 

 illuminated field. 



The number of operations that must be carried out 

 before the whole apparatus is in adjustment for critical 

 light may seem unnecessarily tedious for simple work 

 which does not really require such care, but, if the base- 

 board or optical bench has been properly constructed, 

 the microscope, lamp, condensers, and camera will be 

 found to be almost exactly centred. Only very small 

 final adjustments should be required, and these are so 

 quickly effected that there is no need to omit them at any 

 time. 



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