26 THE AMERICAN MONTHLY [Febniary, 



thickness of cover-glasses is most apparent ; but since it is so, we should, 

 if possible, provide an agency which, eliminating the personal factor of 

 efficiency, will give, under all conditions, results closely equal to those 

 under which the objectives were originally corrected. 



It is surprising to see how little attention is paid to this subject in the 

 large majority of standard works on the microscope. Almost all books 

 give carefully-prepared illustrations and descriptions showing the effect 

 on the course of light by the interposition of the cover-glass, and after 

 giving conclusive evidence of its disturbing influence, still, in a general 

 way, say it is of little moment. Thus, in a German work of the highest 

 standing, which has also been translated into the English language, is 

 found the following utterance, freely translated : 



"In regard to modern microscopes, which we have had opportunity 

 to examine, we have not found the difference in thickness such as oc- 

 curs in commercial cover-glass, when, for instance, three to six ai'e equal 

 to a mm., has any noticeable influence on the microscopical image." 



In another work of great popularity are found the following quota- 

 tions : " That the effect of thickness of cover-glass has a great influence 

 on the perfection of the microscopical image is beyond the slightest 

 question, and certainly deserves the most careful attention of the optician 

 as well as the observer ; but whether the devices of its removal are of 

 such great importance and so absolutely necessary as it is claimed, is 

 another question. On the other side, the diflerence in the cover-glass 

 used in different directions for the most delicate preparations is hardly 

 of any account. I at least possess, besides my individual preparations 

 covered with glass of about 1-5 mm. thickness, a collection of objects 

 which I obtained from London and Paris, in which there is such a slight 

 diflerence of cover-glass thickness that I can observe them all with my 

 objectives of powers from 2.0 to 1.3 mm. (equivalent to about 1-13 to 

 1-20 inch) without showing the slightest difference in optical qualities 

 and in the definition and clearness of the image under the same illumi- 

 nation, as I have convinced myself by careful comparative tests." 



With such statements to guide the microscopists, it is not surprising 

 that the subject should have received so little attention, and that any 

 efforts to lead to improved methods of manipulating objectives should 

 have almost completely failed because of a lack of the true understand- 

 ing of their need and consequent failui'e to create intei'est. The belief 

 is quite general that any time devoted to this subject is wasted and might 

 better be utilized in other directions. I hope to be able to show that 

 this is entii'ely wrong, and may here say that, while I maybe considered 

 an extremist in the other direction, my efforts emanate from the desire 

 to put it in the power of every microscopist to obtain the highest possi- 

 ble results from his optical battery and equal to those obtainable by the 

 optician. 



When in 1887 Prof. S. H. Gage addressed a circular letter to all 

 opticians in the world inquiring for the dimensions of their standard 

 tube length, as well as for the thickness of cover-glass which they used 

 as a standard in the correction of objectives, I looked forward to the 

 result with considerable interest, as it would bring together data which 

 it was impossible to obtain otherwise. 



At the meeting of this society in 1887, at Pittsburgh, he gave the 

 I'esults of his efforts, which show some astonishing facts. I would here 



