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Ox A New and Simple Method of Micrometry. 

 By John Matthews, M.D. 



(Read August 21th, 1869. 



I WISH this evening to lay before you and describe a new 

 and simple method of micrometry. I had some idea of calling 

 it " a rough and ready method," but am scarcely justified in 

 that, since it really is capable of considerable precision. Of 

 this I leave you to judge for yourselves, when I tell you that I 

 have by its aid counted the markings in Pleurosigma formosum 

 and angidatum easily, and that this has been done not only by 

 myself, but by a person totally unaccustomed to microscopy, even 

 of the most ordinary kind. 



Perhaps I ought to preface my description by stating that, in 

 my opinion, it is incumbent on every one who seeks to introduce a 

 new method into the pursuit of a science, or the practice of an art, 

 to justify it by stating what he conceives to be the defects or in- 

 conveniences of the existing methods, and then to show how he 

 proposes to remedy them. 



The errors, then, which may result from the present modes, are 

 of three kinds. Istly, from their principle. 2ndly, from the 

 faulty construction of the instruments used. 3rdly, from inac- 

 curate observation or manipulation. And here I will acknowledge 

 that there is no doubt that very good, nay, the best possible, work 

 has been done in the present way by experienced observers ; yet it 

 is surely most desirable that even the tyro should have at his com- 

 mand the greatest possible power of ensuring precision, with the 

 least chance of error. It is scarcely necessary to speak to an 

 audience like this of the present modes of micrometry, since they 

 are well-known, and as well described in the pages of the admir- 

 able manuals of Carpenter, Hogg, and others, whose treatises are in 

 every one's book-case. Suffice it to say, shortly, that their general 

 application has been to the eye-piece, where Dr. Carpenter affirms 

 that they may be most advantageously used; but I venture to 

 think this at least doubtful, as I shall, by-and-bye, endeavour to 

 show you. 



