36 



THE AMERICAN MONTHLY 



[February, 



angular aperture, which is expressed 



by ^ = — •) '^ being: the wave-leneth 



of the light, it is obvious that the 

 limit of resolution for any aperture 

 depends upon the length of the vi- 

 brations. In the Numerical Aper- 

 ture Table which is published from 

 time to time in this journal, among 

 the advertisements, the value of 

 A is 0.5369//, which is the wave- 

 length of yellow light, and the theo- 

 retical limit of resolution given in 

 the table is only true for light of 

 that particular color. Taking the 

 wave-length of blue light corre- 

 sponding to the spectrum line f, 

 i ^ 0.48606;/., which would give a 

 smaller value to () in the equation, 

 hence the theoretical limit of resolu- 

 tion would be materially increased. 

 In photographv the actinic rays are 

 assumed to be most active between 

 lines G and h in the spectrum, hav- 

 ing the maximum action near line 

 h^ ^ = 0.4000,'/. 



According to the table referred to, 

 the theoretical limit of resolution for 

 yellow light, or what may be re- 

 garded as the limit for ordinary- white 

 light, for the highest possible nu- 

 merical aperture, which is 1.53, is 

 146,528 lines to an inch. Using 

 monochromatic blue light the num- 

 ber rises to 158,845, and in photog- 

 raphy the limit rises to 193,037, cor- 

 responding to line // in the spectrum. 

 Mr. Stevenson has calculated the 

 limits for various apertures, and em- 

 bodied them in a table for the joumial 

 referred to. 



It will be understood by the reader 

 that the theoretical results cannot be 

 fully realized in practice. 



More lines can be resolved with 

 svnilight than with lamp-light, but 

 this is not due to the preponderance 

 of short vibrations, as has been quite 

 generally supposed. The short vi- 

 brations act upon a photographic 

 plate with greater intensity than the 

 others, but in eye-observations they 

 are not strong enough to be effective 

 against the brighter portions of the 



spectrum. Hence with sunlight we 

 have only the power of resolution of 

 white light, as given in the table, but 

 owing to the intensity of the light it 

 is possible to utilize the extreme 

 angular aperture of an objective, and 

 thus approach more nearly to the 

 theoretical limit. This portion of 

 the subject is fully explained in the 

 article i^eferred to. 



Microscopical Societies. — We 

 have once more opened a column to 

 be devoted to reports and notices of 

 microscopical societies. The Wash- 

 ington vSociety desiring to have its 

 proceedings published, we were glad 

 to ofler such space as can be spared 

 for the purpose, and at a recent 

 meeting the Secretary was requested 

 by the Society to send regular re- 

 ports to the Journal. 



Hereafter all news relating to the 

 societies will be published in the col- 

 umn now established for the pur- 

 pose, and we believe it will prove an 

 interesting part of the Journal. 



The Washington Microscopical 

 Society, although one of the young- 

 est, is having good meetings, and 

 there is always something brought 

 forward, either in papers or discus- 

 sions, that is worth recording. We 

 receive regularly reports from the 

 San Francisco Society, which have 

 been noticed as often as possible, and 

 no doubt other societies will in fu- 

 ture send occasional if not regular 

 notices. 



NOTES. 



— Messrs. Walmsley & Co. have just 

 issued the seventeenth edition of their 

 fully illustrated Catalogue of Microscopes 

 and Accessories. It is the most complete 

 microscopical catalogue to be obtained, 

 embracing the manufactures of Messrs. 

 R. & J. Beck, Bausch & Lomb and other 

 makers. The lithological stand deserves 

 to be well known as a convenient instru- 

 ment for a moderate price. The new 

 Star microscope is a ^15.00 stand, de- 

 scribed recently in these columns, well 

 deserving the praise bestowed upon it. 



