NA TURE 



[May 24, 1894 



THE THEORY OF OPTICAL /.VSTRUMENTS. 



Tkeorie tier Ofitischn-'t Instrumcntt {nach Abbe). Von 

 Dr. Siegfried Czapski. (Breslau : Eduard Trewendt, 

 1S93) 



TTAPPENIXG not long ago to meet a German friend 

 i ^ well posted up in physical literature, the present 

 writer inquired whether any of the year's publications 

 were specially worth getting and reading. The answer 

 was a do-.:blful '' No,'' and then " Oh, yes — Czapski's 

 ' Theory of Optical Instruments.' " This was high praise, 

 but not unmerited. For although the book will not 

 appeal to a large circle of readers, it will soon become 

 indispensable to all who are interested in the investiga- 

 tion of the merits and defects of optical systems, or 

 who are concerned in turning out high-class optical 

 work. 



Although only a third of the book is devoted to the 

 description of the microscope, telescope, and other in- 

 struments, there is no unnecessary rambling beforehand 

 — either into the pleasant regions of developmental 

 history or along the more dusty paths of the optical text- 

 book proper. The author acknowledges the value of the 

 undulatory theory as the ultimate test which must be 

 applied when we wish to know how far the conclusions 

 of geometrical optics are valid ; and therewith dismisses 

 it. He everywhere tries to be concise ; and succeeds so 

 far that the average student might complain of sometimes 

 finding a day's work between one page and the next. 

 But the condensation is not of that meaningless kind 

 which so often irritates one in German school and college 

 text-books. There are frequent (and valuable) biblio- 

 graphical notes. The references to the works of Smith, 

 Herschel, Lloyd, Airy, Rayleigh, Dallinger, and Pendle- 

 bury show that the English literature has not been 

 neglected ; indeed, Principal Heath's " Treatise on Geo- 

 metrical Optics" (which is being translated into German) 

 is recommended as the best of the modern text-books. 

 But whereas these tend to treat optical instruments as 

 convenient illustrations of optical theory or geometrical 

 reasoning, the latter are here treated as strictly subservient 

 to the former. The range is further limited by including 

 among optical instruments only those which are strictly 

 used for producing images of external objects applica- 

 tions of reflecting mirrors in geodesy and astronomy, the 

 stereoscope, &c., being included. Within these limits 

 our author is at home and speaks with authority. As 

 scientific adviser and technical director in the celebrated 

 workihops of Carl Zeiss in Jena, he has daily oppor- 

 tunities of applying theory to practice with the aid of 

 excellent glass, skilful workmen, and modern machinery; 

 and has had the still greater advantage of continual inter- 

 course with Prof. Abbe, to whose labours the perfection 

 of high-power microscope objectives is so largely due. His 

 debt in this direction is freely and gratefully acknowledged 

 — most conveniently by stating what chapters are not 

 directly or indirectly due to Abbe. 



A short introductory chapter on geometrical optics 



is followed by three others on the geometrical theory of 



optical images, the fundamental properties of lenses and 



systems of lenses, and the theory of spherical aberration. 



NO, 1282, VOL. 50] 



In chapter v. (on chromatic aberration and the theory o 

 achromatism) the author gives a simple method for cal- 

 culating the magnitude of the secondary spectrum (the 

 focus-diflference of a system of two infinitely thin lenses 

 from the dispersion-constants. Assuming these to be 

 achromatised for the lines F and C, he calculates the 

 difference between the focus for light of a given wave- 

 length (X) and the light of the brightest part of the 

 spectrum X = 055 n. (.According to Konig the position 

 of maximum brightness in the spectrum of sunliglu 

 varies, as the total intensity increases, from X = 053 fx to 

 X = 061 ik) The values of the differences /a -/.jj^ are 

 given in thousands ofyi jj u for various wave-lengths. In 

 the caseofa combination of a tlint glass of medium densits 

 with an English silicate-crown glass the differences iXt-- 

 crease from + 179 (for X = 077^) and then gradu.1i; 

 increase to 3 70 (for A = 041 »i). But the researches 01 

 Prof. .Abbe and Dr. Schott have resulted in the produc- 

 tion of improved grades of optical glass, and especialU 

 of pairs of tlint and crown glass in which the dispersion 

 in various parts of the spectrum is much more neailv 

 proportional. These glasses have been manufactured in 

 the Jena works since 1SS4, and by their use the ma, 

 nitude of the secondary spectrum is greatly diminishc ;. 

 Thus in the case of a combination of a heavy barium 

 phosphate-crown glass with a borate-flint glass the above 

 differences begin at - 004 (forX = 077 /j) and tlie maxi 

 mum value is 079 (for X = 041 fi). Curves are givia 

 illustrating these results which have been conlirmed by 

 direct measurements of focus-differences of telescope- 

 objectives made by \'ogel, Hasselberg, and Wolf. 



Chapter vi. contains an unusually complete and sys- 

 tematic treatment of prisms and systems of prisms, partly 

 based upon Dr. Czapski's own investigations. The next 

 chapter is devoted to stops and aperture, and the pro- 

 perties of an optical system which depend upon apertuic. 

 such as penetrating power and brightness. It is shown 

 that in any instrument used for subjective observation 

 the penetrating power {i.e. the total depth of vision) i^ 

 exactly equal to the sum of the depth of focus of the ob- 

 jective and the accommodation-depth of the eye — a resuli 

 which is of the greatest practical importance in micro- 

 scopy. Next comes what is perhaps the most interestini; 

 and important chapter — that which deals with tin 

 principal types of optical instruments. The book closi 

 with an account of the methods for determining the 

 constants of optical instruments. Some of these will 

 come as a revelation to students who are only f.imili.ii 

 with the comparatively rough methods described in tin 

 ordinary text-books on practical physics. 



Dr. Czapski appears to have at lirst intended toir.clul' 

 in the book a tolerably full account of .Abbe's theory ■ 

 microscopical vision. One cannot help regretting that tli 

 intention was abandoned. A letter of his on " The Futuir 

 of the Microscope," which is printed in van lieurck's 

 sumptuous book ("The Microscope,'' English edition, 

 PP- 357 3'^'4) shows his ability to present such a matter, 

 not only with his usual accuracy, but in a fairly popular 

 form. We understand th.at a separate volume on this 

 subject may be expected from I'rof. Abbe or Dr. Czapski, 

 or both. pv. 



