314 HOW TO SEE WITH THE MICROSCOPE. 



" If ordered for English microscopes the lens will be 

 perfectly corrected for a tube of ten inches exactly, and 

 no sensible deviation from this length would be ad mis- 

 sable without a loss of perfect definition. However,, 

 owing to the slight defect of refractive power in the 

 oil of cedar-wood, some advantage may be found by 

 lengthening the tube for one-half to one inch while ob- 

 serving through extremely thin covers (less than 0.004 

 inch), and by shortening it for one-half to one inch in 

 the case of very thick covers (exceeding 0.008 inch).* 



" The object-glass is made with fixed brass work and 

 with standard screw (like all my object-glasses) ; the 

 price of it is 240 marks ; the price of the one-twelfth 

 is 320 marks ; the aperture guaranteed to be not less- 

 than it is stated above. The lenses are screwed together 

 with moderate pressure and may be unscrewed without 

 great effort, but I caution expressly against unscrew- 

 ing them ; owing to the great aperture the system is- 

 extremely sensible to the slightest defect of centering 

 the smallest particle of dust, or the least moisture get- 

 ting into the screws, and the unavoidsible difference of 

 pressure when screwing the lenses together, would 

 cause a sensible loss in the performance of the glass." 1 



JENA, March, 1878. (Signed) CARL ZEISS. 



One can hardly read the foregoing somewhat contra- 

 dictory document without arriving at the conviction 

 that Mr. Zeiss has suffered severely at the hand of his 

 translator. Be this as it may it will be noticed that he 

 asserts that the oil-immersions practically NEED adjust- 



* Italics mine, J. E. S. 



