390 HOW TO SEE WITH THE MICROSCOPE. 



Now, after seeing and learning what can be accomplished 

 with a superior objective, when out of adjustment, it will be 

 instructive, and I shall beg your permission, by the employ- 

 ment of a superb glass, accurately adjusted, to show you a sim- 

 ilar, but more difficult tiaxoaica, as illuminated by the most 

 oblique beams my extremely thin stage will admit. Thus han- 

 dled the resolution of Frustulia Saxonica becomes one of the 

 most charming and fascinating objects that can well be imag- 

 ined. We shall thus see the frustule without sensible distortion 

 the striae displayed with such exquisite beauty of definition as 

 must command your admiration, and minus, too, the suspicion 

 of a diffraction line I 



The attention of Andrew Ross, while examining Podura 

 scales with glasses of his own manufacture, was called to the 

 thickened edge, the " Nasmyth membrane " so plainly seen with 

 the non-adjustable glass he then employed, and to him are we 

 indebted for the collar adjustment now so common to all first- 

 class objectives. 



Now, my friends, good object-glasses, like astronomers, have 

 their " personal equation." They alone are the ones to be most 

 affected by collar .adjustment ; an ordinary glass, furnished 

 with compensating screw, is scarcely superior to an ordinary 

 non-adjustable lens ; one may turn the collar through its entire 

 range, without sensible or corresponding change in definition. 



me saying that he was surprised that Dr. Woooward should have allowed 

 the publication of prints giving such indifferent ideas of the work of 

 American lenses. I replied, that as I then understood the mat er, it was 

 Col. Woodward's intention to show the work of the objective purposely 

 placed out of adjustment, and I so continued to think at the date of the 

 above lecture, a fact obvious to the reader. I also suggested that the litho- 

 graphs might not fairly represent the photographs ; that if it was the case, 

 Col. Woodward would surely make the fact known. His silence, however, 

 authorized the inference that the original photos had not suffered at the 

 hands of the lithographer. I th nk I can now safely affirm that the general 

 opinion is that one of these lithographs was intended by Col. Woodward to 

 give a correct idea as to the appearance of P. S.ixonica when properly 

 resolved, i. e., that it might be contrasted with the others then presented. 

 Be all this as it may, I am prepared to asse:t that no one of said lithographs 

 gives any idea of the proper resolution of Fr.istulia Saxonica. J. E. S. 

 January, 1878. 



