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CRITICAL ILLUMINATION IN VISUAL WORK WITH! 



THE MICROSCOPE. 



By Duncan J. Keid, M.B., CM. 



{Bead April 2Zrd, 1912.) 



When I was asked if I would read a paper on " Illumination in 

 Visual Work with the Microscope," it seemed to me that to do 

 so to the members of this Club would be almost superfluous. 

 On further consideration, however, there occurred to me several 

 reasons why, after all, the reading of such a paper might not be 

 taken amiss. First of all, there are always a few beginners to 

 whom it might be useful. Then again, there are many 

 experienced microscopists who have not the time to devote to 

 inquiries on the technique of illumination, who might even learn 

 something, either from .the paper itself, or from any discussion 

 which might follow on it. Lastly, it must be admitted that there 

 are still a good many disputed points even in connection with 

 such a well-worn subject as illumination, and I thought that a 

 discussion on these might be both interesting and useful. 



I have confined my paper to the consideration of illumination 

 in " critical work,'' by which I mean decisive or exact work. 

 Every earnest microscopist has, at times, problems to solve, 

 which demand all the perfection of apparatus and of technique 

 that is obtainable for their solution, and perfect illumination is 

 one of the most important factors in such cases. 



I should like, however, in the first instance, to refer to a few 

 precautions which enhance the effect of good illumination. All 

 unnecessary and extraneous light should be avoided, and there- 

 fore one should have only as much light in the room as is 

 absolutely necessary, and should certainly not face a window in 

 daylight. The lamp should be shaded, so that no direct light 

 from it is to be seen except that passing through the microscope. 

 The observer should save as much as possible his best eye, when 



