Flat-Wavelet Resolution. 591 



have been resolved before it entered the objective, u e. as it 

 existed in the oil space between the cover-glass and the 

 objective. It is the resolution of this light which is exhibited 



to us by the concentration image. 



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Fiff. 14. 



The part of the indicator diagram made visible by an immersion 

 objective *. 



43. It is well known to microscopists that in working with 

 objectives of large aperture, it is necessary to adjust with 

 care the correction-collar when the objective has one, or else 

 the tube length ; and that with dry objectives this adjust- 

 ment will vary with the thickness of the cover-glass and of 



the mounting material through which the light has to make 

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its way before reaching the objective, and with immersion 

 objectives it will depend upon the index of refraction of the 

 mounting material, and upon its thickness, in other words, 

 upon the interval between the object and the under surface 

 of the cover-glass. The reason of this becomes apparent by 

 studying the concentration image. We may conceive that 

 image divided into annuli, and the microscopical image (D, in 

 fig. 11) ought to be formed by the satisfactory cooperation 

 of the cones of rays coming from these several annul i. Now, 



* A« represents the entire indicator diagram ; B6 is the part of it 

 made visible by an objective of NA=1'35; !>'// is the disk of light 

 which each punctum of the flame can furnish if an immersion condenser 

 of na=l*3 is in satisfactory adjustment. CV is the similar disk 

 furnished by a dry condenser of n« = 0'9. when in adjustment. 



