Intelligence and Miscellaneous Articles. 73 



manner in which the phenomena of magnetic intensity, and of the 

 dip of the needle at different latitudes could be explained on the 

 hypothesis of an earth's internal magnetic shell, such as here de- 

 scribed, was then pointed out ; and the analogy of such a magnetic 

 shell with a magnetic bar passing through the centre of the earth 

 was illustrated. 



The author then proceeded to account on geo-dynamical principles 

 for the secular variation of the magnetic needle, and also to show 

 how the objections that might be raised to the views here advanced, 

 on the grounds of the high temperature which must be assumed to 

 exist at the depth beneath the surface of the magnetic magma, 

 could be met by considerations of pressure, and on this subject read 

 a letter which he had received from Sir William Thomson, F.B-.S. 



In conclusion, the author stated it was impossible in a short 

 abstract to go into the details of the subjects here discussed, and 

 for further information the reader must be referred to the paper 

 itself. 



NOTES ON METALLIC SPECTEA. BY C. C. HUTCfllNS. 



In the work herein described an attempt has been made to deter- 

 mine the wave-length of several metallic lines with something of 

 the precision with which wave-lengths of solar lines are known and 

 tabulated. 



It has been repeatedly pointed out that wave-lengths of metallic 

 lines from the determinations of the best observers are liable to 

 errors of one part in 3000 or 4000 ; while Rowland has given us 

 the position of a long list of solar lines correct to one part in 

 500,000. It is too often forgotten that Thalen used a single bisul- 

 phide-of-carbon prism in his researches, and that consequently his 

 places can in no sense be considered standards of precision for the 

 more powerful instruments of the present time. 



The spectroscope employed in the present work has a large flat 

 grating with ruled space 5 by 8 centim. Upon the margin of this 

 grating Professor Rowland has written : " Definitions exquisite." 

 The collimator and view-telescope are combined in a single lens, an 

 excellent objective by Wray, 6 inches in diameter, 8| feet focus. 

 The radius of curvature of the back surface of this lens equals its 

 focal length, so that the ray reflected from this surface passes back 

 to the slit, and any objectionable illumination of the field is avoided. 

 All parts of the instrument are so contrived that it is operated 

 without the necessity of the observer leaving his seat at the eye- 

 piece. A heliostat and achromatic lens of 5-feet focus form an 

 image of the sun upon the slit. Thus arranged, the instrument 

 easily performs all tests of spectroscopic excellence with which the 

 writer is familiar. To produce the metallic spectra an 8-inch 

 spark, condensed by a number of jars having about six square feet 

 of coated surface, has been employed. The spark is produced 

 immediately before the slit, the jaws of which open equally. The 

 coil is operated sometimes by a dynamo, and sometimes by the 

 Phil. Mag. S. 5. Vol. 28. No. 170. July 1889. G 



