204 ON THE THEORY OF THE MICROSCOPE. 



The persistence of empirical practice is commonly attributed to 

 the technical difficulty, or supposed impossibility, of maintaining 

 the requisite accuracy in executing the prescribed measurements 

 of the several constituent lenses of a microscope objective.* At 

 first sight this explanation seems plausible enough, since the 

 minuteness of dimensions, especially in the case of high power 

 objectives, may well cause the difficulty of working such measure- 

 ments to the required nicety to appear almost insuperable. 

 Nevertheless, the objection does not apply; on the contrary, a 

 careful consideration of the scientific and technical means at the 

 disposition of the practical optician, and a critical comparison of 

 the various kinds of difficulties serving as a guiding thread and 

 key to the theoretical discussion of the conditions influencing 

 them, have led me to the conclusion (supported since by actual 

 successful results) that lenses and systems of lenses of which each 

 separate part has prescribed dimensions, can be thus executed 

 with an exactitude that fairly ensures correct action, and with 

 greater facility than any other mode of procedure offers for the 

 fulfilment of the same conditions with equally good results ; and 

 consequently it only needs that the calculations for each separate 

 element of the optical effect should be correct to secure, with good 

 execution, the success of a given theoretical construction. 



In the workshops of T. Zeiss, of Jena, the construction of the 

 various objectives, from lowest to highest power, is regulated by 

 strict calculation (based upon accurate analysis of the material) 

 for each single part, each curve, each thickness of glass, each 

 degree of aperture; so that all guess work and ''rule of thumb" 

 is avoided. The optical constants of each piece of glass used are 



of such researches as form the subject of Dr. Abbe's essay, or a stronger 

 argument for extending their publicity, if only for no other reason than to 

 uphold the independence, and reassert the dignity of pure science after its 

 almost forced abdication and temporary weakness. 



♦ Throughout this essay the German terms " Objective" and " Ocular" 

 are retained as more suitable than the equivalent English terms " Object- 

 glass" and "Eye^piece." 



