202 Note. 



work. The workmanship is exceedingly delicate and difficult, especially 

 as to the front lens. Among the twenty-four opticians in Mr. Z.'s 

 workshop there is only one who can grind a front lens admitting a 

 balsam angle of 113°, and only one who can make the brass- work to it. 

 We were obliged to make several new contrivances for the work before 

 it succeeded. 



The sensibility of the system, in point of chromatical correction, ■ 

 for the dispersive power of the immersion-liquids, is an agreeable cir- 

 cumstance in one respect. By the nature of the optical glass, flint and 

 crown, as it is till now, there is, unavoidably, a difference of spherical 

 aberration, for different colours, and, from this cause, a difference of the 

 colour-correction between the central and the marginal parts of a system. 

 Now this difference can be easily regulated by slightly changing the 

 dispersive power of the immersion-liquid ; a smaller dispersive power of 

 this liquid will give the best correction of colour for oblique light — 

 a greater dispersion for central light. 



In a similar way, slight differences in the length of tube may be 

 perfectly compensated by a different mixture of the fluid. 



In two or three weeks, I hope, the new lens will be in your hands ; 

 and I think you will be contented with the result, your impulsion has 

 brought forth. 



In the meantime I send you my best wishes for a happy new year, 

 and remain 



Yours sincerely, 



(Signed) E. Abbe, 



The above letters, of course, are of very great interest to micro- 

 scopists from the historical point of view, but they have in addition, 

 at the present time, a peculiar interest for opticians and optical 

 manufacturers generally, because of the information they give 

 indirectly as to the high state of efficiency which had, even at that 

 time, been reached at Jena in the work of optical designing and 

 computing. We know that at this time Abbe had set himself 

 " to abolish once and for all workshop trial-and-error methods, and 

 to replace the rough approximations to the ray-path by more exact 

 mathematical determinations, which, expressed in cast-iron formulae, 

 should enable the necessary structural elements of the lenses, such 

 as thicknesses, diameters, radii of curvature, etc., to be exactly 

 determined by calculation simply and alone." 



It will be noticed that on November 14, 1877, the calculation 

 for the proposed objective had been completed " some days ago." 

 Six weeks later only. Professor Abbe was able to announce that 

 his mathematically determined results had been realized in a 

 concrete construction with great exactitude. We are told that 

 the experiment with the oil-immersion objective had been suc- 

 cessful ; the computed aperture had been exactly realized without 

 any lack of definition. This is a lesson which should be taken to 

 heart at the present time. Many people are under the impression 



