The President's Address. By Frof. P. Martin Duncan. 179 



corrected objectiyes or witli those which have an average adjust- 

 ment. I pointed out in my last address how abnormally thick, 

 slender and excessively minute bodies appear under a high amph- 

 fication ; this is partly due to a want of correction, and mainly to 

 another cause which it is not necessary to revert to. Now I have 

 no hesitation in saying that similar abnormalities are constantly 

 recorded as truths, and for that same reason which causes excellent 

 observers to differ in a most remarkable manner about the appear- 

 ances of the same object under different Microscopes. 



Many microscopists do not know how to use the screw-collar, 

 and I have taken the pains to inquire of several workers how they 

 proceed to correct. A very general answer is, I focus the dirt on 

 the top of the thin glass and then screw down until the object 

 is in focus. So that this mistaken correction produces a double 

 amount of error. 



No one doubts the necessity for correcting dry and water- 

 immersion objectives, or that it is very much less in the case of 

 homogeneous-immersion systems, but in spite of Dr. Dippel's 

 lively assertions to the contrary, practice has shown that an object 

 whose true shape has been learned has been seen all the better after 

 correction for the index of refraction of the thin glass cover with a 

 medium above it and one below. 



Doubtless this refraction is minimized by the homogeneous- 

 immersion system, but even then there is the variable refractive 

 index of the thin glass cover to be considered. 



For perfect work, correction is necessary in the instance of the 

 oil-immersion objectives. 



Unfortunately the method of correcting, or rather the amount 

 of approach of the front and back set of lenses of the objective 

 which is required, has to be estimated empirically. Either the 

 front lens and back pair are to be placed at a medium distance, and 

 when the objective is focussed on the object this distance must be 

 diminished or increased until what is believed to be the true shape 

 of the object is seen; or what has the greater rudiments of 

 exactitude in it, the focus having been taken with the lenses 

 sufficiently distant for uncovered objects, the collar is worked to 

 the left until the top of the thin glass is seen. Then the object is 

 refocussed. This last plan would be the best were it not a fact 

 that a source of error is revealed by the increase of amplification 

 during the correction. The method of Mr. Wenham, by which the 

 hinder pair of lenses is pushed forward towards the stationary distal 

 lens, in correcting for cover, is vastly superior to the old plan of 

 moving the front lens. 



What exact relation there should be between the distance of the 

 front and back series of lenses in the objective, and the thickness of 

 the thin glass cover over the object, and that of the medium between 



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