CHAPTER VI 



EXPERIMENTS ON ILLUMINATION 



Critical Light. The correct illumination of an object 

 is of the utmost importance when a high degree of resolu- 

 tion is required, and the use of the condensers needs even 

 more study than that of the objectives and eyepieces. The 

 object to be attained is a strict correlation of the two 

 optical parts of the microscope, the illuminating and the 

 image-forming, and their exact combination in one com- 

 plete optical system. This is the most difficult problem to 

 be faced in photomicrography, but no high-class work can 

 be done unless it is understood and practised. The follow- 

 ing experiments are intended to pave the way to its 

 complete mastery, and to serve as an introduction to the 

 proper use of the radiant and condensers, and to the 

 procedure by which the best illumination is obtained. 



It will be well to start with a concrete conception of 

 the problem, and for that purpose definitions of " Critical 

 Image " and " Critical Illumination " are given in the 

 words of Mr. E. M. Nelson, the protagonist of that method 

 of illumination for microscopical research : 



Critical Image. " The image of an object is critical 

 when it is obtained by means of an objective of fine 

 quality which has been placed in correct adjustment for 

 that object by its screw collar, or by the alteration of the 

 tube length, and when the illumination is critical." 



Critical Illumination. " An object is said to be 

 illuminated critically when it is placed at the apex of a 

 solid axial cone, the aperture of which is not less than 



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