Note. 199 



• — the product of refractive index, with the sine of the semi-aperture 

 angle being = 1*25 (while with the immersion lenses of Zeiss till now 

 this product is = I'lO). The technical work of such a system will be 

 somewhat difficult, but I hope we will succeed. In a few days the lenses 

 will be ground, and as soon as the thing is finished, I will send it to you 

 and ask your judgment. 



There is a difficulty in the combinations with oil-immersion, which 

 will be observed as well in the technical work, i.e. the fabrication, as in 

 the application of such an objective, and which, perhaps, will make 

 illusory this advantage, which you make prominent : the independence 

 of cover-thickness and the dispensing of a correction-setting. The want 

 of a medium of different refraction in front of the system will take away 

 the most effective element for compensating the slight differences of 

 workmanship in the lenses (the unavoidable differences of curvature and 

 thickness) and the differences in the length of tube. By want of this compen- 

 sating element the objective will be most sensible for the least difference 

 in the length of tube ; and, I presume, a sort of correction-setting will 

 be necessary in this respect, unless the use of the lens should not be 

 restricted to a tube of exactly defined length. Now the experiment 

 will soon settle this point. 



As to the liquid, a mixture of oil of turpentine with oil of cloves will 



do very well, and it will be very easy to any observer to get the 



right mixture by observing the disappearance of a piece of cover-glass in 



the fluid, which is a very sensible proof for equal refractive index. . . . 



* * * * « * 



Repeating my excuse for the long delay of my answer, I send you my 

 best compliments, and remain 



Yours most sincerely, 



(Signed) E. Abbe. 



The second letter reads as follows : — 



Jena, Deer. 29, 77. 

 Dear Sir, 



I have the pleasure to give you notice, that the experiment with 

 the oil-immersion objective has been successful. Some accidents in the 

 work and a great pressure of business in Mr. Zeiss' workshop during the 

 last months, have been the cause of a longer delay than I expected. 

 But these difficulties have been overcome, we have got a lens of excellent 

 performance. The aperture taken in my calculation — 1 • 25 numerical 

 value = 113° balsam-angle — has come out exactly without any lack of 

 definition. The objective will bear extraordinarily deep eye-pieces, and 

 in accordance with the calculated effects will give a very flat field. As 

 I have no great experience in fine diatom work, I did not make many 

 trials of this kind. But the advantage of the great aperture is already 

 visible in a most striking manner on the image of Pleurosigma angulatum 

 observed with oblique light. This diatom mounted dry (but with 

 frustules adhering to the cover) will give quite a new aspect, if the 

 oblique pencil of light is directed perpendicular to the axis of the 



