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SUPPLEMENT. 361 



now be totally valueless for advanced work. We now demand, 

 as near as may be, perfect lenses, and superior dexterity in 

 handling them ; and these two conditions are inseparable. The 

 finer the objective the louder the call for expert manipulations. 

 That well-known term, u working lenses of narrow angles" 

 means, when stripped stark naked, easy going lenses, with no 

 screw collar to bother ; good working lenses, that a child or 

 sleepy adult without experience can use right along, will work 

 through covers of common window glass, big working distance, 

 and all that, etc., etc. Such are admirably adapted to the use of 

 those who use the microscope as a plaything ; admirable things, 

 too, to prove that " a little knowledge is a dangero.is thing." 



Be it known, that I do not condemn an objective simply 

 because it has a narrow aperture ; conversely, I do not endorse 

 a glass on account of its wide angle. I have seen scores of 

 wide angle glasses not worth the cost of their brass mountings. 

 As to * 4 errors in interpretation," the more perfect the lens, and 

 the more expert the manipulator, the less chance of error.* 

 Under high amplifications, a superior wide-angled glass, properly 

 handled, will generally prove the more reliable ; ami in advanced 

 work cannot be dispensed with, be the illumination central or 

 oblique. 



Finally, I have to thank Mr. Hitchcock for his friendly criti- 

 cism. He seems, evidently, to be after the facts. I have re- 

 sponded to his request as well as I could with my limited time 

 and space. Two hours "over the tube" would demonstrate 

 more than volumes of print. 



,T. EDWARDS SMITH. 



Note. On my last visit to the Dunkirk Society, May, 1877, 1 

 showed, I believe, for the first time, the Robert 19th band as an 

 opaque object, with my Tolles' one-tenth duplex, Beck's vertical 

 illuminator being used. It is obvious that the pencils of light 

 traversing that band were at least centrally disposed. J. E. S. 



ANGULAR APERTURE AND CENTRAL ILLUMINATION. 



In my reply to Prof. B. Hitchcock, reprinted in the August 

 number of this journal, I endeavored to respond to his court- 



