166 



NATURE 



[May I, 1919 



medium, so dense that there are very many mole- 

 cules per cubic wave-length — for example, in the atmo- 

 sphere—must arise from the thermal motions of the 

 molecules rather than from irregularity of their spac- 

 ing. In directions, however, that are nearly coincident 

 with the transmitted ray there can be no sensible 

 dispersal of phase from either cause ; disturbances, 

 therefore, completely conspire, and the light scattered 

 by the molecules in such directions is, in Lord Ray- 

 leigh's phrase, specially favoured. Prof. Raman 

 points out that if the phases in directions near that 

 of the ray did not in fact thus agree, the molecules 

 of the material medium could take no concordant part 

 in the transmission of the energy of the main beam, 

 and regular propagation would be impossible. It is 

 involved in this remark that each molecule will exert 

 its full effect on the index of refraction, however ir- 

 regular the distribution may be, provided it is not so 

 dense that the molecules will obstruct each other; and, 

 moreover, the thermal motions will not disturb this 



INDIAN ASTRONOMICAL INSTRUMENTS.' 



INDIAN astronomy, handed down to us in a 

 series of text-books, the Siddhdntas, of w^hidh 

 the earliest dates from about a.d. 400, is an ofif- 

 spring of Greek astronomy. Via Babylon and the 

 Greek kingdom of Bactria (the Kabul valley and 

 the Punjab), Greek science was introduced into 

 India in the course of the two or three centuries 

 following the invasion of Alexander the Great. 

 While, during the Middle Ages, many astronomers 

 in western Asia and North Africa, did good work 

 by re-determining astronomical constants and im- 

 proving planetary tables by new observations, no 

 attempts whatever in this direction were made in 

 India. It is therefore very curious to find that an 

 extremely belated effort to revive the study of 

 astronomy, and at last to try to advance this 



-General view, Jaipur Observatory. F 



effect. The specially favoured directions, for disturb- 

 ances passed on by the molecules, must be almost 

 coincident with the ray — must, in fact, belong to the 

 ray after the manner of diffraction, when it is regarded 

 as a physical filament of light rather than as a 

 geometrical line. 



The light that may be scattered in a crystal must, 

 on any view, be due either to motes embedded in it 

 or to the thermal motions of its molecules around 

 their regularly spaced mean positions. A beautiful 

 recent experiment by Prof. Strutt, now reported in 

 the Proceedings of the Royal Society, exhibits a spiral 

 beam scattered sideways when plane-polarised light 

 passes through a column of quartz ; thus, incidentally, 

 it puts in direct visual evidence the slow rotation of 

 the plane of polarisation of the vibrations which arises 

 from the spiral structure of the crystal. 



Joseph Larmor. 



Cambridge, April 24, 



NO. 2583, VOL. 103] 



science by independent work, was made early in 

 the eighteenth century in the north of India. 



Rajah Jai Singh of Jaipur (born 1686, died 

 1743) was interested in astronomy from his youth. 

 He wrote or caused to be written an astronomical 

 work named after the Emperor Muhammad Shah, 

 of which there are now extant an imperfect copy 

 in Sanskrit (at Jaipur), and a complete MS. in 

 Persian in the British Museum. It contains a 

 star catalogue, which, however, is nothing but 

 the catalogue of Ulugh Beg, with 4° 8' added to 

 the longitudes to allow for precession. A transla- 

 tion of the introduction was published by Hunter 

 in the "Asiatic Researches," vol. v. (1799); it is 



1 "The Astronomical Observatories of Jai Singh." By G. R. Kaye. 

 (Archaeological Survey of India, New Imperial Series, vol. xl.) Pp. viii-f- 

 151 + 26 plates +1 map. (Calcutta, 1918.) Price 23^. 



