NATURE 



409 



THURSDAY, MARCH 



THE EVOLUTION OF ASTRONOMICAL 

 INSTRUMENTS. 



Ziif (Jcschichtc der astrouomischen Mcssi\.'crkzcugc 

 wn Piirbach his Reichciibach 1450 bis 1830. By 

 Joh. A. Repsold. Pp. viii+132. (Leipzig:: Wil- 

 lielni Engelmann, ic,o.S.) Price 16 marks. 



IN all that relates to the mounting of telescopes or 

 the construction of instruments intended for 

 accurate measurements, in all that increases their 

 convenience or adds to their efficiency, the firm of 

 Kepsold has won a world-wide reputation, and the 

 booi^ before us indicates in some measure the reason 

 for this marked success. The head of the firm has 

 been a keen and interested student of the history of 

 past construction. For more than forty years, the 

 author reminds us, he has been engaged in furthering 

 the progress of instrumental construction, and in this 

 lime he has given close study to all that has effected 

 the gradual development of this branch of engineering 

 technique. He has assimilated all that experience can 

 teach, has learnt the strength and the weakness of 

 th':; work of past masters, and has profited by their 

 example and their attainments. We now in our turn 

 have the opportunity of benefiting by the results of 

 this close study, perfected by much examination and 

 sifting, and in addition to tracing the evolution of 

 modern instruments we get glimpses of the history 

 of .astronomy, viewed from sl new and interesting 

 standpoint. Obviously, the connection between the 

 progress of astronomical science and the improvement 

 in instrumental equipment must be continuous and 

 intimate, but how close the tie is can hardly be appre- 

 hended until we make a historical survey of the prin- 

 ciples of instrumental construction, on a plan which 

 reveals the part played by successive makers, and 

 niai<es us understand to what extent astronomy has 

 been forwarded by their endeavours. 



\llhough the author limits his review from 1450, 

 whin Purbach strove to give expression to his 

 nuclirinical ideas, to 1830, when Traughton in Eng- 

 l.Lud and (jambey in France were representatives of 

 (he art of instrument making, the survey cannot be 

 restricted to precise dates. At one end we listen again 

 to the description of the contrivances of Ptolemy, 

 wlilch served for models through so manv centuries, 

 and at the other we are permitted to see the begin- 

 nings of the famous house of Repsold, destined to 

 influence the methods of future artists. 



The author passe> in review the mechanical efforts 

 of the .\rabians. w hose claims to consideration have 

 been extravagantly championed bv Sedillol and as 

 stoutiv disputed by Delambre. He acknowledges the 

 skill of the devices which enabled them to solve 

 approximately a particular class of problems by 

 mech.mical means, but cannot find much to praise in 

 their measuring instruments. The astrolabe and the 

 so-c,-illed " sextant " meet with adequate recognition, 

 but a careful consideration of the facts, which are set 

 out u ith the clearness bespeaking the practical expert, 

 le.-ivts the impression that the .\r.abians did verv little 



NO 200 r, VOL. 77] 



to advance the means for making accurate observa- 

 tion. They imitated, they did not invent, and none 

 of the generic improvements which have facilitated 

 the subdivision of small intervals of time and space — 

 the main problem which has engaged the attention 

 of successive generations of instrument makers — on 

 which accurate astronomy depends, can be placed to 

 their credit. 



Frequent reproduction has made us familiar with 

 the forms of the instruments used by Copernicus and 

 Tycho, who with Hevel may be regarded as the 

 last representatives of a pre-telescopic age. But here, 

 in addition to very complete illustration, we have from 

 the pen of a competent authority a full technical de- 

 scription of those contrivances, accompanied by acute 

 and illuminating remarks on the adequacy of the 

 design to secure the end contemplated, the faults of 

 construction, and oftentimes the reason for the adop- 

 tion of particular methods. Down the stream of time 

 this discriminating but generous criticism is pursued, 

 necessarily affording a clearer insight into the diffi- 

 culties and successes of individual artists. 



The introduction of the telescope offered a new set 

 of problems for solution. The continual increase in 

 focal length compelled makers to abandon the sector 

 form of instrument, such as the quadrant, and forced 

 upon them the necessity of devising more appropriate 

 means for measurement, though Halley and Bradley 

 both used S-feet quadrants. The employment of 

 complete circles and the designing of instruments of 

 greater symmetrv in their several parts were the 

 consequence, and no one displayed more ingenuity 

 or foresight than did Romer. With justice, the author 

 carefully discusses the work of this astronomer, whose 

 claims to recognition have been very tardily admitted, 

 mainly owing to Delambre's jealous partisanship of 

 Picard. But Dr. Repsold has known how to do 

 justice to the one without injury to the other. Romer 

 in various ways anticipated modern design. His 

 inachina domestica was the prototype of the 

 present transit instrument, his rota meridiana of 

 the meridian circle, while his azimuthal instrument 

 foreshadowed the introduction of the convenient uni- 

 versal instrument. The use of two bearings to carry 

 a long axis of rotation, increased symmetry of struc- 

 ture, the adaptation of the reading microscope, the 

 practice of determining instrumental errors by suitable 

 mechanical means, were all as fully appreciated by 

 Romer as they are to-day. 



Considering how indispensable a micrometer is to a 

 telescope, and how materially it increases the scope 

 and usefulness, its evolution proceeded slowly, but 

 the study of its many transformations is of singular 

 interest. The urgency of the demand for the means 

 of measuring the diameter of a planet seems out of 

 proportion to its importance, but the solution of the 

 general problem, containing as it does that of the 

 accurate and convenient subdivision of small spaces, 

 taxed the ingenuitv of instrument makers severely. 

 Huyghens proposed a thin wedge, which could be 

 moved in the focal plane until the planet was exactly 

 occulted, when the measurement of the breadth of 

 the wedge at that point gave the diameter. A net- 



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