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SCIENCE 



[N. S. Vol. XXVI. No. 658 



so valuable a member of the board. As he has 

 all along been one of the most obsequious 

 supporters of the president, having absolutely 

 no will of his own, the grounds of his value 

 are evident. To the credit of the faculty be it 

 said, that most of them refused to be 

 " grafted " for such a purpose. As the in- 

 stitution was founded to promote " religion, 

 morality and knowledge " it is evident from 

 what appears above and from much additional 

 testimony that might be adduced that these 

 terms are just now somewhat " liberally " 

 interpreted. 



Chas. W. Super 

 Athens, 0., 

 July 22, 1907 



SPECIAL ARTICLES 



IMPROVEMENTS IN THE ULTRA-VIOLET 

 MICROSCOPE 



The resolving power of a microscope varies 

 directly as the numerical aperture of the ob- 

 jective and inversely as the wave-length of the 

 light employed." In other words, the shorter 

 the wave-length the smaller the objects that 

 can be distinguished. Light of half the usual 

 wave-length will show details one half the 

 size of those seen with ordinary light. 



The advantage of using light of extremely 

 short wave-length for microscopic purposes has 

 been known for many years and was given 

 clear expression by Czapski in 1891.° For 

 some time, however, little or nothing was done 

 to carry out Czapski's suggestions, for several 

 reasons. First, ultra-violet light is invisible 

 to the eye and though able to affect the photo- 

 graphic plate energetically, can not be focused 

 directly even on a fluorescent screen inserted 

 in the camera in place of ground glass, on 

 account of the weakness and indistinctness of 

 the image when high powers are used. 



" This is commonly expressed by the formula 

 (i=:X/24 where d^size of smallest detail re- 

 solved by the microscope, X = the wave-length of 

 the light employed, and A = the numerical aper- 

 ture of the objective. 



' Czapski, S., Die voraussichtlichen Grenzen der 

 Leistungsfahigkeit des Mikroskops, in Zeitschr. 

 f. tviss. Mikroslcopie, 8: 145-155, 1891. 



Second, the glass of which ordinary objectives 

 are made is opaque to all but the relatively 

 long waves of ultra-violet light which lie just 

 beyond the visible spectrum, which rays give 

 but slightly increased resolution. So little 

 advantage could be gained that glass ob- 

 jectives corrected for ultra-violet light were 

 never made. 



Early in the present century, Kohler began 

 exiDerimenting with lenses of quartz and 

 fluorspar, two substances very transparent 

 to ultra-violet light. Such lenses could be 

 used with ultra-violet of very short wave- 

 length which would give greatly increased 

 resolving power. 



While Kohler was in the mids* of these 

 experiments von Rohr, in 1902, made a great 

 discovery. He invented a new system of 

 lenses made of only a single substance, yet 

 almost perfectly corrected for spherical aber- 

 ration for light of a certain definite wave- 

 length. 



Herschkowitsch shortly before had learned 

 how to malve optically homogenous melted 

 quartz in fragments large enough for the 

 minute lenses of a microscopic objective. 

 Under Kohler's energetic leadership, these dis- 

 coveries were utilized at once and within two 

 years he was able to describe a complete outfit 

 for using ultra-violet rays in photomicro- 

 graphy and to publish numerous plates show- 

 ing the remarkable performances of this new 

 ultra-violet microscope." 



These new lenses, called monochromats, are 

 corrected for ultra-violet light of one definite 

 wave-length — a bright line in the spark spec- 

 trum of cadmium whose wave-length is 0.275 ^, 

 or as more commonly written, 275 fji/j.. With 

 ordinary light composed of many wave-lengths, 

 the images given by the monochromatic object- 

 ives are distressingly bad, blurred and fringed 

 with rainbow colors due to chromatic aberra- 

 tion, for which the lenses are not at all cor- 

 rected. It is out of the question to focus the 

 object with such light, and the statement pub- 



^ KShler, Aug., Mikrophotographisehe Untersuch- 

 ungen mit ultra-violettem Licht, in Zeitschr. f. 

 iviss. Mikroslcopie, 21: 129-165, 273-304, Figs. 

 1-8, Pis. 1-6, 1904. 



