ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 619 



microscopic objects, furnishing the best proof of the coincidence of 

 the optic and actinic foci. The eye-piece is removed and the camera 

 placed in position, without having to make use in any way of coloured 

 or subdued light for the illumination. 



In an abstract * of Prof. Zenger's papers by G. Fischer, he expresses 

 the apprehension that the unavoidable changes of temperature to 

 which the lenses would necessarily be subject would be likely to im- 

 pair their efficiency, and adheres to his own view that absolute 

 achi'omatism will in all probability only be obtained by the discovery 

 of more favourable kinds of glass. 



Prof. Zenger subsequently wrote f to Herr Fischer that Merz's 

 crown glass is still much wanting as regards refraction and disper- 

 sion ; in his view crown and flint never give a rational dispersion, 

 although flint containing different quantities of lead approximates 

 to it. Incomparably better is the achromatism obtained by his fluid 

 lenses, which are as much in advance of the best achromatics of the 

 present time as these latter are in advance of the non-acbromatics. 

 The analogy of the eye, which formerly led to the discovery of 

 partial achromatism, prompted him to try and obtain absolute 

 achromatism by imitation of the gelatinous fluids of the eye, that is 

 by mixing two, three, and four different fluids. In this he has 

 succeeded; two or three fluids, oil and balsam mixed, give, compared 

 with crown glass or quartz, quite rational spectra ; that is constant 

 ratio of the partial dispersions. A constant dispersion-quotient can 

 be obtained for the whole length of the spectrum within 0*002 to 

 0-004, therefore much better achromatism than with the best of 

 Merz's systems, in which the quotients differ from 0*004 to 0*026. 



Finally he points out that the experiences of photograj)hy suffice 

 to show how much the best productions of the first modern opticians 

 fail in collecting all the rays to one focus. He, on the contrary, is 

 able with his fluid system to obtain micro- and astrophotographs 

 without the interposition of coloured glasses or adjustment-correction, 

 just as if his lenses were mirrors ; consequently, all rays, chemical and 

 optical, are united in one focal point. 



Prof. Safarik has pointed out J to Herr Fischer that whilst with 

 Zenger's objectives perfect achromatism is undoubtedly almost attain- 

 able, yet it is very doubtful whether aflanatism (removal of spherical 

 aberration) is also attainable. With Merz the diminution of the 

 dispersion-relation necessarily entails a lengthening of the focus, the 

 reverse of what opticians have hitherto striven to obtain. " Whether," 

 adds Herr Fischer, " Zenger's system, the three-lens system (Merz's), 

 the improved Herschel-Fraunhofer system with more perfect kinds of 

 glass, Pliissl-Littrow's, or an entirely new system, attains the desired 

 end, this much may, I consider, be confidently expected, that sooner 

 or later a considerable improvement of the achromatism, and with it 

 of the optical capacity of the Microscope and telescope, will be assured. 

 In conclusion, I gladly avail myself of the opportunity of bringing 



* Central-Ztg. f. Optik u. Mcch., iv. (1883) pp. 254-6. 

 t Ibifl., p. 2(37. 

 X Ibid. 



2 T 2 



