182 THE AMERICAN MONTHLY [June, 



EDITORIAL. 



Spoiling Lenses. — Unfortunately a fifty dollar lens may be 

 spoiled in ten seconds. The very carefulness to wipe it is suf- 

 ficient to spoil it if rubbing is indulged in with gritty paper or 

 cloth. A lens must be cleaned but not by rubbing. Rub1)ing 

 generates electricity and the electrified glass attracts dust. New 

 velvet may be used but some soft skin like chamois is safest. 



Heat and cold must be avoided for they cause expansion or 

 contraction of the mounting and may produce dis])lacement of 

 the lenses. 



Solar light acts upon all kinds of glu^s ; it imparts a slight 

 coloring that has been studied by Faraday, Bontemps, Fresnel 

 and others. It is rarely noticed in optical glasses but when dis- 

 covered, the lenses may be worthless in case their properties 

 have undergone change. 



Moisture must be guarded against for it readily condenses on 

 tlie glass when present, being attracted by the large proportion 

 of alkalies used in the manufacture of the glass to impart trans- 

 parency. 



Czapski's Theory of Optical Instruments. — A well in- 

 formed German physicist, when asked if any book had beea 

 printed during the year of special value, having at first said no, 

 upon second though said : Oh, yes. — Czapski's Theorie der Opti- 

 schen Instrumente — (Nach Abbe). This was unintentionally a 

 very high compliment and calls for some notice. The writer 

 shows perfect familiarity with the writings of Herschel, Smith, 

 Lloyd, Airy, Rayleigh, Dallinger, Pendlebury, etc. Practically 

 he had the great advantage of practicing upon all his thories in 

 the great works of Zeiss in Jena where he is scientific adviser 

 and technical director. His intercourse with Prof. Abl)e, with 

 skilled workmen, and his access to the best machinery and ma- 

 terials, all conspired to educate him for this task. His article 

 in Van Heurck's last book on " The Future of the Microscope " 

 introduced him to us in a pleasing manner, and his visit to the 

 Columbian Exposition served to make his name familiar to 

 American microscopists. 



His first chapter is on geometrical optics ; the second and 

 third and fourth on the geometrical theory of optical images, the 



