330 



NATURE 



[May 13, 1920 



each other; what is observed is, then, the transit 

 of the two images over the same line of a 

 horizontal graticule. 



There is no appreciable difference in precision 

 between the two prism arrangements. B has the 

 apparent disadvantage that a closer adjustment of 

 the telescope is required, the line of collimation 

 must be perpendicular to the prism base, and the 

 latter must be truly vertical, whereas with A the 

 horizontality of the telescope and the symmetrical 

 inclination of the prism faces are immaterial. On 

 the other hand, from the practical surveyor's point 

 of view, the use of the easily damaged silvered 

 faces is inexpedient, and the method of internal 

 reflection preferable. The disadvantages attach- 

 ing to the necessity of more careful adjustment of 

 telescope and prism are, moreover, more apparent 

 than real. It must be remembered that while the 

 actual observation involves no reading of gradu- 

 ated circle or micrometer, a horizontal circle is 

 required for the purpose of directing the line of 

 sight, so that the desired star will cross the field. 

 The telescope and circle must therefore be levelled 

 and adjusted as with a theodolite, and the extra 

 labour involved in the setting of the prism is a 

 very small matter. 



As already stated, the preparation of an observ- 

 ing programme involves somewhat lengthy com- 

 putations. These have now been made, and are 

 available for the use of observers within a wide 

 range of latitude. The "Handbook of the Pris- 

 matic Astrolabe " gives a succinct description of 

 the smaller survey form of the instrument, its 

 construction and method of use, and contains 

 tables of all the Nautical Almanac stars down to 

 the fourth magnitude which cross the altitude 

 circle at azimuths suitable for observation for 

 each degree of latitude between 55° S. and 55° N. 

 This list gives sufficient stars for all field work 

 except geodetic survey of the first order, for 

 which more and fainter stars would be wanted. 

 For these, reference must be made to the "Astro- 

 labe Diagram," giving, for the same limits of 

 latitude, a series of graphs from which the azimuth 

 and time of any star crossing the altitude circle 

 can be plotted. A comparison of the relative 

 accuracy of the astrolabe and other survey instru- 



ments seems to indicate that it is probably 

 capable of somewhat greater precision than a 

 theodolite of similar telescopic power; and there 

 is no doubt that in it we have a valuable addition 

 to the resources of the surveyor. It cannot, how- 

 ever, take the place of the theodolite, being cap- 

 able of determining only latitude and time, not 

 azimuths or angles. It has therefore been urged 

 as an objection to its more extended use that as 

 a survey party must in any case carry theodolites 

 the astrolabe could be taken only when the added 

 labour of transport is unimportant. Apart from 

 the fact that the addition of thirty pounds to the 

 baggage of a survey expedition would be found 

 burdensome only in quite exceptional cases, this 

 objection does not appear to have any validity. 

 A theodolite is capable of conversion into an astro- 

 labe by the addition of the prism and mercury 

 trough, and it would be easy to design these so 

 that they could be clamped on to the front of the 

 telescope, and the prism levelled in a minute or 

 two. The extra weight would then not exceed 

 a few ounces. 



An attempt, has been made, not, however, yet 

 carried very far, to develop the use of this instru- 

 ment for the astronomical problem of the deter- 

 mination of star places of high-order precision. 

 It is very doubtful if it presents any real advan- 

 tages for this work. The difficulty of making true 

 plane surfaces is well known, and in an instru- 

 ment of large aperture and high magnification the 

 inclusion of flat reflectors in the optical system is 

 undesirable. Furthermore, the two star images 

 are not symmetrical, each being formed by only 

 half the object glass, and the results show a 

 magnitude equation, or variation with the bright- 

 ness of the stars observed. This has not been 

 specially studied in the portable survey patterns, 

 but would probably be found even with them. 

 Triimpler (be. cif.), using an aperture of only 

 47 cm. and a focal length of 50 cm., found it 

 conspicuously. It would increase rapidly with 

 increase of aperture. For the present we must 

 regard the astrolabe as a surveyor's instrument, 

 capable in his hands of useful service, and leave 

 any possible application to observatory work for 

 further investigation. E. H. H. 



The Heart of 



By Douglas 



CENTRAL ASIA " used to conjure up in the 

 imagination thoughts of lonely and mys- 

 terious frontiers between three great Asiatic 

 Empires, of strange doings in unheard-of valleys 

 on the Pamirs, of long-dead conquerors, and of 

 strange capitals at the back of the world. Even 

 now, in 1920, the heart of Asia is a storm centre, 

 for it forms the meeting-place of the civilisations 

 of the remote past — China ; of the present — Great 

 Britain; and of the future? — Bolshevism. 



Great happenings have been in middle Asia — 



1 "Through Deserts »nd Ogses of Central Asia." By Miss Ella Sykes and 

 Brig. -Gen, Sir Percy Sykes. .Pp. xii + 340. (London : Macmillan and Co., 

 Ltd., igao.) Price ixs. net. 



NO. 2637, VOL. 105] 



a Continent.! 



Carruthers. 



unheard-of movements, unimagined miseries— 

 during the past six years, when all men's thoughts 

 have been concentrated on Europe and the Middle 

 East. The remote highlands and deserts of Asia 

 did not escape the turmoil. The most secluded 

 and most apathetic native races felt the ripples 

 of the storm in Europe. The confines of China, 

 India, Russia, and Afghanistan have returned, by 

 a strange coincidence, to their former place itS, 

 what may well be, the centre of a prolonged 

 struggle, not between East and West, but between 

 right and wrong. 



Chinese Turkestan, or Kashgaria, is that 



