80 THE AMERICAN MONTHLY [March 



Tests For Microscope Objectives.}" 



Hv i:i)WARI) M. NKLSON. 



LONDON, ENl'-KAND. 



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

 lution of diatomic striae witli oblique light — eyepieciug 

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

 jective has also (contrary to the usually received opinion) 

 litle to do with it; a bad objective may be a strong stri* 

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

 aperture, skillful manipuhition, 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 [tractice. 



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 strije by oblique illumination is no criterion 

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



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

 good test for the highest quality lenses from :^in. up- 

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

 slide." Asa I'ule, it is ]>etterto 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 tiie objective, 



tin reply to the following (lucstions : (1) lor what particular powers are 

 the following diatoms generally recognised a.s suitable tests: Surirella gemma, 

 rieurosigma attenuatum, I'leurosigraa augulatnm. N'avicnla lyra, Gram- 

 iiiatophora marina, Stauroneis plidMiiceuteron, Triceratium favus? (2) In 

 which of tlie following media are the alK)ve diatoms resolved most easily with 

 divohjectivcs of suitable power and aperture: Styrax, balsam, mono brom 

 naphthalin, mono-bnim balsam, or mounted dry? C?) What is approximately 

 the lowest magnilyinu iM)wer under which, ^vitli an objective capal)le of 

 dividing Plcurosigma angulatum, the dots may be distinctly discerned by an 

 eve of average power of vision? i4) Which variety of Coscinodiscus most 

 easily shows the secondary markings? 



