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 admis- 

 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 unavoidable difference of 

 pressure when screwing the lenses together, would 

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



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. 



