NATURE 



THURSDAY, SEPTEMBER -•<>. 



APPLIED OPTH v 

 Applied Optics: The ('imputation of Optica 

 Systems. Being the "Handbuch der ange- 

 wandten Optik " ol Dr. Adolph Steinheil and 

 Dr. Ernest Voit. Translated and edited by 

 |. Weir French. Vol. i. Pp. xvii + 170. 

 (London: Blackie and Son, Ltd., i<)iS.) Price 

 !-•>. >hI. net. 



TIIT first volume of Steinheil' s handbook 

 appeared twenty-eight years ago, and the 

 promised second and third volumes of the work 

 never materialised, probabl) owing to the first 

 meeting with insufficient appreciation. The book 

 before us is a translation of the first half of Stein- 

 heil's first volume, and the fact that a prominent 

 member ol one oi our foremosl optical firms (Barr 

 and Stroud) considered ii worthy of this labour 

 is eloquent poof of the truth of a statement by 

 the late Prof. Silvanus P. Thompson in a noted 

 outburst : 



"The simple reason of the badness of almost 

 all recent British text-books of optics is that . . . 

 they are written, not to teach the reader real 

 optics, but to enable him to pass examinations 

 set bj non-optical examiners. The examination- 

 curse lies over them all." 



Steinheil's book certainly does not belong to 

 this category ; it is severely practical and almost 

 crude in its empiricism. Scarcely any of the 

 numerous formula; given in the book are proved; 

 the reader must cither accept them and mechani- 

 cally follow the scheme of the numerous numerical 

 examples, or he must discover the proofs by his 

 own effort. In the case of the complicated Seidel- 

 formulae for rays not proceeding in a plane con- 

 taining the optical axis, a student unfamiliar with 

 modern spherical trigonometry is not likely to 

 succeed in this, and the proof of these formula;, 

 together with a clear explanation of the adopted 

 method of astronomical computation with angles 

 up to 360°, should certainly have been included 

 in this first volume of the translation instead of 

 being relegated to the promised second volume. 



The Steinheil system of symbols is safe, but 

 cumbersome, on account of the multitude of suf- 

 fixes; the use of precisely the same symbols for 

 paraxial and for marginal rays is, however, likelv 

 to cause confusion. The sign-' onventions agree in 

 all ordinary cases with those almost universally 

 adopted by practical computers; the only defect in 

 them is that all the signs are made to depend on 

 the direction in which the light travels through 

 the system; hence if the latter includes reflecting 

 surfaces — a case expressly and nei essarily included 

 in the scheme — the signs of all the angles and 

 intersection-lengths must l>, ■ \ nsed before pro- 

 ceeding to the following surface. This complica- 

 tion is entirely avoided if the direction of the light 

 is ignored and axial intercepts arc given the sign 

 usual in analytical geometry, and if the acute 

 angles between the optical axis and the ray are 

 2552, V( d . I02 ! 





6l 



( died positiv 1 



ise turn. 



The worst feature 

 the definitions of tli 

 are not only loose, 

 incorrect. Thus on 





u -e ponding with a i loi 



is to be fourftjSln 

 ■' nations, wh/ch 

 Hit als,, [ieuiajpt J ly..p6Si>rVely 

 p. 45 the iuiucrtjajit^-mrTe-con- 

 dition is merely implied — and only in the form 

 which it takes for systems applied to infinitely 

 distant objects. The condition is correctly stated 

 in its general form on p. 57, but the st : men< is 

 immediately vitiated by the assertion that it is 

 fulfilled "when the system has the sam 1 



length for any portion of the whole aperture" — . 

 i.e. when it is fulfilled for infinitely distant 

 With rare exceptions, in the case of certain 

 systems having great thickness or wide sep 

 tions, the exact contrary is true : A system fulfil- 

 ling the sine-condition for objects at infinitv do< 

 not fulfil it for objects at finite distances. 



The worst confusion of this kind occurs in the 

 case of distortion. On p. 44 this is correctly, 

 although loosely, defined in its accepted meaning. 

 Throughout the rest of the book the term is used 

 for the defect universally known as coma, simplv 

 because the latter, by diffusing the rays over a 

 certain area, necessarily causes most of them to 

 fall away from the position of the ideal image- 

 point. Steinheil thus ignores the fact that true 

 distortion may exist in an otherwise perfect image, 

 and that it causes a linear displacement of any 

 image-point which is proportional to the third 

 power of its distance from the optical axis, whilst 

 the coma-displacement is proportional directly to 

 the distance of an image-point from the optical 

 axis, and also to the square of the aperture of the 

 image-forming cone — which latter has no effect 

 at all on true distortion. 



On p. 56 coma is described as "spherical 

 aberration out of the axis," which, again, is 

 wrong; true spherical aberration may exist in 

 oblique pencils independently of that on the optical 

 axis, but it is a fifth-order aberration which has 

 nothing to do with coma. 



There are many other cases of a type similar to 

 the above examples. 



The book is beautifully printed on paper of 

 extraordinary thickness, and the translator and 

 editor may be congratulated on the excellence of 

 his part of the work. A. E. C. 



THE MEGALITH I C CULTURE OF 

 INDONESIA. 



The Megalithic Culture of Indonesia. By W. J. 

 Perry. Pp. xiii + 198. (Manchester : At the Uni- 

 versity Press ; London : Longmans, Green, and 

 Co., 1918.) Price 12s. 6d. net. 



IN his presidential address to Section 11 

 Nature, vol. lxxxvii., p. 356), at the n 

 of the British Association at Portsmouth 1 1 191 1, 

 Dr. Rivers explained how he had been led to reject 

 the popular dogma of "spontaneous generation*' 

 hnology, which is wrongly claimed to be 

 evolution," and to realise the vast importance 



