July, 1906.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



479 



Photoheliograph. — Photographs of the sun were obtained 

 on 210 days. The Thompson telescope was taken to Sfax 

 for the total solar eclipse, and excellent photographs of the 

 solar surroundings obtained. By the co-operation of other 

 observatories photographs were obtained for measurement of 

 spots on facute on 364 days during 1905, the missing day 

 being 1905, January i. 



Maqndic Observations. — The principal results for 1905 are 



as follows : — 



Mean Declination = 16° g'-g West. 



T-, • , T, (40173 (British Units.) 



Mean Horizontal Force = J^„ ;,. . ., .. , 



1 1-8523 (Metric Units.) 



Mean Dip (with 3-in. needles) = 66^' 55''55". 



In 1905 there were no days of great magnetic disturbance, 

 and only 12 of lesser disturbance. 



Personal. — During the year Mr. F. VV. Dyson, the chief 

 assistant, was appointed Astronomer-Royal for Scotland, 

 and the vacancy was filled by the appointment of Mr. A. S. 

 Eddington, B.A. 



In concluding his report the Astronomer makes some 

 strong remarks concerning the serious interference with the 

 work of the observatory which is caused by the new power 

 station of the London County Council, situated on the river- 

 side, due north of the transit circle, and only about half a 

 mile distant. The tremors caused by the engines are stated 

 to be much more violent than would have been anticipated 

 from experience of the engineering plants that have been 

 carefully tested. 



Stereoscopic Determination of Stellar 

 Proper Motions. 



Herr Dr. Max Wolf, Director of the Astrophysical Ob- 

 servatory at Heidelberg, has for some years past very 

 successfully employed the stereoscope for detecting asteroids 

 on photographic star charts taken at intervals, and he has 

 recently extended this delicate method of comparison to the 

 determination of proper motions of stars perpendicular to the 

 line of sight. The process involves the operation of com- 

 paring, in an ordinary stereoscope, two photographs of the 

 same portion of the sky taken at several years' interval. 



In the case of plates of a well-known star submitted to 

 the Paris Academy there is a distinctly visible alteration in 

 the appearance of the star from its surroundings, although 

 the two component photographs were obtained at an interval 

 of only four years. Another comparison refers to a star of 

 the ninth magnitude in Leo, whose proper motion now be- 

 comes known for the first time. The two plates for show- 

 ing this were taken at an interval of fourteen years. 



Not only is the detection of proper motion rendered much 

 more easy by this method, but Dr. Wolf is of opinion that 

 its magnitude can be determined more accurately than by 

 the usual micromctric method. 



As examples of other problems to which the stereoscopic 

 method of comparison may with advantage be applied, men- 

 lion is made of the examination of meteors, comets, the 

 lunar mountains, and certain nebulae (Comptes Bendus, 142, 

 pp. 1007-8, May, 7, 1906. 



Parallax of Nova Persei. 



M. O. Bergstrand has recently ende.ivoured to determine 

 the parallax of the Nova I'ersei from observations made at 

 the observatory of Upsala in 1901 and 1902. 



His result is 



p - o"oj ± 0"'0I 



Distribution of Radium in the Earth's 

 Crust. 



An important paper dealing with the results of an in- 

 vestigation of the mode of occurrence of r.-idium in the 

 earth's crust has been communicated to the Royal Society 

 by the Hon. R. J. Strutt. He concludes that : (i) Radium 

 can easily be detected in all igneous rocks. Granites, as a 

 rule, contain most radium ; basic rocks the least. (2) This 

 distribution of radium is sufficiently uniform to enable a 

 fair estimate to be made of the total quantity in each mile of 

 depth of the crust. (3) The result indicates that the crust 

 cannot be much more than 45 miles deep, for otherwise the 

 outflow of heat would be greater than that actually ob- 

 served. The interior must consist of totally different 

 material. This result agrees with Professor Milne's con- 



clusion drawn from a study of the velocity of propagation of 

 earthquake shocks through the interior. (4) The moon 

 probably consists for the most part of rock, and, if so, its 

 internal temperature must be far greater than that of the 

 earth. This explains the great development of volcanoes on 

 the moon. (5) Iron meteorites contain little, if any, radium. 

 Stony ones contain about as much as the terrestrial rocks 

 which they resemble. — (Proc. Roy. Soc, 77A, pp. 472-485, 

 1906.) 



New Method for the Discovery of Asteroids. 



.-Vn interesting method has recently been adopted by J. H. 

 .Metcalf for the photographic determination of asteroids. 

 It may, perhaps, be best described as the inverse procedure 

 to that adopted by Max Wolf at Heidelberg, where the star 

 field is accurately followed by clockwork, and any asteroids 

 present record themselves as short trails among the numer- 

 ous minute points representing the stars. When the 

 asteroids are in opposition they retrograde about 34" an 

 hour, and thus in an instrument of large dimensions the 

 trails must of necessity be somewhat faint ; therefore a 

 limit is soon reached to the brightness of asteroids which 

 will be detected by the trail. The author, therefore, calcu- 

 lates the mean velocity of the asteroids in the region it is 

 proposed to examine. By inspection of the positions and 

 daily motions given in the Berliner -Jahrbuch, it is possible 

 to get a close value of the extreme values for the motions 

 of the asteroids already known in a certain region. The 

 general direction of motion will be parallel to the elliptic. 

 By means of a finding telescope with micrometer wire the 

 camera is so adjusted that it can be moved at short intervals 

 in the calculated direction ; at the end of the exposure all 

 the stars in the field wdll be represented by short trails, while 

 any asteroids which may be present will most probably be 

 shown as points or onh' very short lines. Excellent photo- 

 graphic reproductions of asteroids found in this way are 

 given, one of about the thirteenth magnitude. The author 

 acknowledges the description of a somewhat similar method 

 of photographing unknown objects which was published by 

 E. E. Barnard in 1897. 



Testing of Optical Surfaces. 



M. G. Meslin describes a \ery interesting method of test- 

 ing optical surfaces which is more widely applicable than 

 the now well-known method of using Newton's rings and 

 monochromatic light. By employing a grating instead of 

 the parallel test plate a new series of interference phenomena 

 are brought into play, and the most important factor from a 

 practical standpoint is that these new bands are very distinct 

 in ordinary white light, so that little special apparatus is 

 needed. The grating is placed over the surface to be tested, 

 either in contact or a few millimetres distant. 



.An interesting distinction of these grating interference 

 bands from Newton's rings is that they are scarcely 

 coloured, being almost achromatic when viewed at an inci- 

 dence about 45" ; further, the diameters of the rings diminish 

 when the incidence is increased, whereas Newton's rings 

 increase in diameter under similar conditions. They may be 

 rendered very brilliant by increasing the reflecting power of 

 the surface to be tested, say, by silvering, and hence the 

 method may be applied to the examination of metallic sur- 

 f.ices, and also of liquids, such as mercury. — Comptes 

 Kendus, 14J, pp. 1,039-1,042, May 7, 1906.) 



BOTANICAL. 



I^y Ci. M.^ssEK. 

 Protective Adaptations in Plants. 



Some remarkable instances of protective adaptation have 

 been described, and beautifully illustrated, by Sir William 

 Thiselton-Dyer, in the Annals of Botany. The plants under 

 consideration are natives of South .Africa, and, growing in 

 an arid region, had two problems to face — to reduce to a 

 minimum the loss of water by transpiration, which was 

 accomplished by assuming a spheroidal form, thus present- 

 ing a minimum of surface ; and, secondly, to secure as far 

 as possible against the danger of such succulent masses 



