May 13, 1920] 



NATURE 



329 



The Prismatic Astrolabe.^ 



T N the process of measuring the places of stars 

 •'• on the celestial sphere, or in the converse 

 process of using these measured places to fix 

 the position of the observer upon the earth's 

 surface, the astronomer has at his disposal two 

 systems of reference lines or circles upon which 

 to base his measurements. These are respectively 

 the vertical great circles through his zenith and 

 the small circles parallel to his horizon, the circles 

 of equal altitude or equal zenith distance. Using 

 the first system, his method is to time the transit 

 of a star across a vertical circle, almost invariably 

 the meridian circle passing through the north and 

 south points. If, in addition to timing the transit, 

 he measures the altitude, he gets a complete 

 determination of the position of the star observed, 

 and uses both sets of reference circles, the vertical 

 circle for fixing the time of transit, and hence the 

 right ascension of the star, and the horizontal 

 circle for fixing the altitude of transit, and hence 

 the star's declination. This is the ordinary 

 observation carried out in the observatory with 

 the transit circle or by the surveyor in the field with 

 the theodolite. Another method of observation 

 which gives the same quantities, though not in 

 the same direct form, is by the use of an instru- 

 ment adapted for the recording of transits across 

 a horizontal circle of constant altitude. An instru- 

 ment of this class is the almucantar, in which 

 horizontality is secured by the device of floating 

 the whole in a mercury bath, it being easily seen 

 that if either the instrument or the bath is moved 

 round, the telescope will maintain a constant angle 

 with the horizontal, and the line of vision will 

 therefore always intersect an almucantar or circle 

 of equal altitude. 



Another instrument of the same fundamental 

 type, but of an entirely different form, is the 

 prismatic astrolabe devised about twelve years 

 ago by MM. Claude and Driencourt. This 

 appears to possess great merits for survey work 

 in the field, and has earned quite enthusiastic 

 praise from those who have used it. The one 

 objection to its more extensive employment, the 

 arduous labour involved in preparing observing 

 lists of stars, has now been removed by the pub- 

 lication of Messrs. Ball and Knox Shaw's "Hand- 

 book " and "Diagram." We will revert to this 

 point later, but we must first give a short descrip- 

 tion of the principles of this interesting instrument. 



It consists essentially of a telescope with a 60° 

 prism in front of the object glass, and a mercury 

 trough placed so as to reflect the star on to the 

 lower face of the prism. 



The prism can be placed in either of the two 

 positions shown in Fig. i, from which it will be 



1 " Description et Usape de I'Astrolabe k Prisme." By Claude et Drien- 

 court. (Paris: Gauthier-Villars, igio.) 



" Bctimmune fundnmentaler Sternflrter aus HShendurchgangsbeobach- 

 tunjfen." By R. Triimpler. Nachrichten der K. G. der Wissenschaften. 

 (GOttingen, 1913.) 



" A Handbook of the Prismatic Astrolabe." By John Ball and H. Knox 

 Shaw. (Cairo : Government Press, 1910 ) 



"Astrolabe Diagram." By John Ball. (Cairo: Government Press, 

 1919.) 



obvious that in both cases, on looking through 

 the telescope at a star which is approaching and 

 near to the altitude of 60°, two images of the star 

 will be seen moving towards each other, and that 

 these images will coalesce into one when the 

 apparent altitude of the star is equal to the angle 

 of the prism. In arrangement A the reflection is 

 from the two outside surfaces of the prism, which 

 must therefore be silvered ; in B we get a total 

 reflection from the two inside surfaces. It is 

 further obvious that with outside reflection the 

 angle of the prism can be given any value; the 

 two star images will always coincide when the 



NO. 2637, VOL. 105] 



—Principle of construction of the prismatic a.strolabe. 



altitude is equal to this angle, whereas if the rays 

 traverse the glass they must enter and leave 

 normal to the faces ; the prism must therefore be 

 equilateral, and stars can be observed only at the 

 fixed altitude of 60°. The observation consists in 

 the timing of the moment of coincidence of the 

 two images. To allow them actually to coincide 

 would, however, render accurate timing difficult, 

 and far greater precision is obtained by giving the 

 telescope a very small lateral displacement, so 

 that the images pass close to, but not exactly over, 



