CHAPTER IV 



THE OPTICAL EQUIPMENT (Continued) 



Sub-stage Condensers, and Illuminating Appliances. It is 



mainly owing to the efforts of English microscopists that 

 sub-stage condensers, for efficiently illuminating microscopic 

 objects by transmitted light, have been brought to their present 

 state of perfection. It was for long considered unnecessary 

 to have any arrangements for so doing that were an essential 

 part of the optical system, and even those that were used were 

 of a crude description, their only function being to get a 

 larger amount of light on to the object. 



It is now recognised that the sub-stage condenser is second 

 only in importance to the objective; and such may be obtained 

 either non-achromatic, achromatic, or apochromatic to suit the 

 particular objective in use. They also vary in focal length, as 

 do objectives, although within narrower limits. 



It is necessary that a condenser should be, at least approxi- 

 mately, as accurately corrected as the object-glass, and should 

 for critical work have as large a N.A. as the objective 

 with which it is being used. In ordinary microscopes the 

 non-achromatised condenser is still supplied for purposes of 

 observation. While it fulfils its purpose well enough for visual 

 work, or for students and others who do not undertake work 

 of a highly critical nature, it should certainty, for photographic 

 purposes, be discarded in favour of a more perfectly corrected 

 combination. It is assumed that, whatever type is used, it is 

 supported underneath the object-stage in the manner already 

 described, and has both focussing and centreing adjustments. 

 If the additional expense of these adjustments in conjunction 

 \\ith an achromatic condenser is an objection, then better 



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