NATURE 



241 



THURSDAY, JANUARY 11, 1900. 



ASTRONOMICAL AND OPTICAL 

 INSTRUMENTS. 

 Handbiich der Astronowischen Instrtimetiienkiittde. Von 

 Dr. L. Ambronn. Zwei Biinde. Mit 1185 in den Text 

 gedruckten Figuren. Pp. vi 4- 1276. (Berlin : Julius 

 Springer, 1899) 

 Die Optischen Instrumente der Firnia R. Fitess deren 

 Beschreibunff, Justieruns;, und Aniuenduna;. V^on C- 

 Leiss. Mit 233 Holzschnitten im Text und 3 Licht- 

 drucktafeln. Pp. xiv + 397. (Leipzig: Wilhelm 

 Engelmann, 1899.) 

 W WHOEVER undertakes to write a manual on astro- 

 V V nomical instruments, or indeed on the instruments 

 that the study of any branch of physics demands, engages 

 in a task of no common difficulty. It is impossible to turn 

 over the two handsome and ponderous volumes that Dr. 

 Ambronn has compiled, without being struck with the 

 wealth and variety of material that is submitted to our 

 notice. . The successive effects of ingenuity as detailed in 

 these volumes are so bewildering in their extent, that it 

 is quite impossible within the limits of a few paragraphs 

 to do justice to the labour and research to which these 

 volumes are an eloquent witness. We can only hope to 

 sketch the scheme, to suggest the lines on which, in the 

 opinion of the author, a text-book on astronomical instru- 

 ments should proceed. The lucidity of explanation and 

 the wealth of illustration only make the task the more 

 difficult, by demonstrating the number of points that are 

 worthy of comment and attention. 



The history of construction, the gradual evolution of 

 the telescope or equatorial, interesting and inviting as 

 such a subject must be, is not allowed to any extent to 

 interfere with the author's project of presenting before 

 astronomers, and before mechanicians, the devices which 

 liave been sanctioned by experience and approved after 

 repeated tests. Historical remarks there must be, the 

 niparison of the work and methods of one maker with 

 L-iother, the growth of convenience and power of instru- 

 ments necessitates descriptions which illustrate historical 

 ])rogress ; but such remarks are incidental, and do not 

 >ncern the main purpose of the work. The present 

 • mdpoint of mechanical art and the achievements of 

 workshops of known reputation, of themselves cover 

 an enormous ground, which will be studied with profit 

 alike by those who seek to modify existing instruments 

 in a direction which will make them available for special 

 investigations, and by those to whom are necessarily 

 entrusted the duties of manufacture and the details of 

 arrangement. 

 The work really consists of seven separate treatises, 

 uh fairly complete in itself, and the whole forming an 

 encyclopaedia, an invaluable work of reference on astro- 

 nomical instruments. These seven sections are entitled : 

 'i) Accessory apparatus; (2) clocks; (3) separate parts 

 of instruments ; (4) micrometers ; (5) instruments devoted 

 to special purposes ; (6) complete instruments ; (7) 

 observatory buildings. The division is somewhat arti- 

 ficial, and cannot be rigorously maintained. For in- 

 NO. 1576, VOL. 61I 



stance, a heliometer might^be described as a complete 

 instrument rather than as a special form of miclometer, 

 and there seems no reason why chronographs should 

 not be treated under clocks. Of course, no confusion 

 can possibly arise from such a method of division, be- 

 cause each instrument serves a definite purpose, and its 

 mode of use is perfectly defined. Simplicity of con- 

 struction is perhaps the best guide to arrangement, and 

 it may be that which the author has followed. Certainly 

 he is well advised in selecting the screw as the first 

 subject for detailed description, and the effective treat- 

 ment applied to this simple piece of apparatus, whether 

 as a tool in the hands of a mechanic or in the more 

 delicate application to measureinent as a micrometer, 

 assures us that we are in presence of a master. We 

 have many useful and ingenious hints both in construc- 

 tion and use, and it is of special importance to notice,- 

 as lifting the book out of the category of merely descrip- 

 tive works, that the author has added an example of the 

 method of determining the errors of a screw as the 

 problem comes before the practical astronomer. But 

 this, and other perfectly legitimate applications of theory 

 to be met elsewhere in the book, suggest a difficulty 

 which, we feel sure, the author has experienced, and 

 introduce a feature which may be considered not al- 

 together satisfactory. How far should a manual of this 

 character concern itself with the theory of instruments ? 

 It was evidently the intention of the author not to supply 

 a descriptive work simply, not a manufacturer's catalogue 

 illustrated by many engravings, but to add also a theo- 

 retical treatise which might be useful to the astronomical 

 student. But to enter into the theory with that rigorous 

 detail which characterises many text-books, would evi- 

 dently carry the author too far, and might add another 

 volume to a work whose length is already sufficiently 

 forbidding. There is, therefore, a constant struggle 

 between the theoretical and descriptive parts, in which 

 the former is usually worsted, and the maintenance of 

 the same high standard of excellence common to both 

 is rendered impossible. For instance, the theory of the 

 sextant is in its way more complete than that of the 

 heliometer, simply because the same actual space is ap- 

 proximately allotted to each. One cannot help feeling 

 that the author has not done himself justice in the matter 

 of theoretical discussion, and we hope that he may be 

 tempted to return to the subject and complete his work 

 by giving a theory as precise and thorough as the 

 descriptive portion is clear and satisfactory. 



From the screw it would have seemed natural to have 

 gone at once to the reading microscope and the micro- 

 meter ; but the author has preferred to interpolate the 

 description of levels, collimators, and other mechanical 

 devices, which are not so much astronomical instrumeixs 

 as aids to adjustment and means for inquiry into the 

 stability of the instrument properly so-called. In the 

 second section, Dr. Ambronn breaks away from his 

 account of space measurement in order to describe 

 clocks and time-recording instruments. The treatment 

 here is mainly conducted from a German point of view, 

 and more attention might have been bestowed on im- 

 provements that have been suggested by English autho- 

 rities. But, of course, one admits readily enough that 



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