80 THE AMERICAN MONTHLY [March 



Tests For Microscope Objectives.f 



Bv KDWARD M. NELvSON, 

 LONDON, ENGLAND. 



Power, practically, has very little to do with the reso- 

 lution of diatomic striae with oblique light — eyepieciug 

 easily remedies any defect on that score; quality of ob- 

 jective has also (contrary to the usually received opinion) 

 litle to do with it; a bad objective maybe a strong strisB 

 resolver. The only other factors left, then, are those of 

 aperture, skillful manipulation, and keenness of percep- 

 tion. Given the requisite aperture, skillful manipulation 

 and keenness of perception (assuming that keenness of 

 vision is present) will come with intelligent practice. 



We must in the first place, recognise that some of 

 the diatoms above enumerated are by no means constant 

 in the fineness of their structure; consequently, the reso- 

 lution of their strife by oblique illumination is no criterion 

 of the aperture of an objective, neither is it of its quality. 



With a 'i axial cone, P. aiigidatum, dry on cover, is a 

 good test for the highest quality lenses from h'm. up- 

 wards. Note, the slide should be what is called a "spread 

 slide." As a rule, it is betterto avoid '^selected diatoms," 

 especially when mounted dry on cover. 



We should also remember that the test lies more in the 

 quality of the image than in the strength of the resolu- 

 tion. Therefore, the quality of an image yielded by a 

 coarse diatom, well within the grip ot the objective, 



flu reply to the following questions : (1) for what particular powers are 

 the following diatoms generally recognised as suitable tests: Surirella gemma, 

 Pleurosigma attenuutum, Pleurosigmu augulatum, Navionla lyra, Gram- 

 matophora marina, Stauroneis ph(pnicenteron, Trioeratium favus? (2) lu 

 which of the following media are the above diatoms resolved most easily with 

 dry objectives of suitable power and aperture: Styrax, balsam, mono-brom 

 uaphthalin, mono-brom balsam, or mounted dry? (.3) What is approximately 

 the lowest magnifying power under which, with an objective capable of 

 dividing Pleurosigma angulatnm, the dots may be distinctly discerned by an 

 t'Ve of average power of vision? (4) Which variety of Coscinodiscus most 

 easily shows the secondary markings? 



