MATHEMATICS AND NATURAL PHILOSOPHY. 451 



this way be referred to the relation in size or in number of tbe vibrations of the 

 light and the particles of the body, and also to the distance of the latter from 

 each other, and as many of the effects are novel in this point of view, it may be 

 anticipated that they will prove of service to the physical philosopher. 



THE SUPPOSED DECENNIAL INEQUALITY IN THE LUNAR-DIURNAL MAGNETIC VARIATION. 



In 1854 General Sabine stated to the British Association that he had at that 

 time found no trace, in the magnetic variations depending on the moon, of the 

 ten-years period which is so distinctly marked in those depending on the sun, and 

 in 1856, after an elaborate discussion of the Toronto observations, he stated this 

 conclusion to be decisively confirmed. M. Kreil having, however, indicated an 

 opinion that the observations at Milan and Prague rather favored the supposition 

 that the same decennial period which exists in the solar variation affects also the 

 lunar magnetic influence, General Sabine, with the unwearying zeal which distin- 

 guishes him, has submitted to analysis the eight years observations at Hobarton, 

 and finds therein confirmation of the conclusions he had arrived at from the 

 Toronto observations, namely, that no such decennial period as Mr. Kreil sup- 

 posed is to be traced in the magnetic influence of the moon, while such a period 

 is indubitably shewn in that of the sun, whether examined by means of the " dis- 

 turbances," or of the " mean " solar variation. With regard to the former General 

 Sabine remarks that, " when the disturbances, occurring at Hobarton during eight 

 years, are broken into four distinct and equal portions, each of two years duration, 

 each such portion manifests the same periodical law of diurnal variation, almost 

 identical in the principal features of direction and turning hours, and differing 

 only in the magnitude of the variation in different years, in which difference it 

 conforms strictly to the decennial period, as indicated elsewhere, having a mini- 

 mum in 1848-44, and a maximum five years later. This law may be accounted a 

 general one, since it has been found to prevail at stations so widely distant from 

 each other as Toronto, St. Helena, and Hobarton." The same result follows when 

 the mean diurnal solar variation, the larger disturbances being excluded, is 

 examined, both for the eight years 1841-8, when the observations were made 

 hourly, and also when the six succeeding years are included, during which two- 

 hourly observations were taken. By treating in the same way the lunar-diurnal 

 variations, General Sabine shows that the differences in these " show no conformity 

 to the inequality manifested in those of the solar-diurnal variations." "With this 

 weight of evidence, we may fairly conclude that the ten years' period has no 

 existence for the moon, and thank General Sabine for the settlement of this vexed 

 question. 



ON THE COMPOSITION OP COLOURS. 



Professor Challis, iu the Phil. Mag., November, 1856, has attempted to give an 

 explanation of this difficult matter, on the principles of the undulatory theory. 

 Taking for his guide the analogy of sound, and a hint thrown out by Sir J. 

 Herschel in his well-known treatise, Professor Challis proceeds to compound two 

 simple undulations of different wave-lengths. As a musical note is produced by 

 a regular succession of similar vibrations, while a mere noise is produced by 

 irregular impulses, so he conceives a simple colour to consist of vibrations, whose 

 type composes only one wave length and amplitude, while a mixed colour, of 

 whiteness, is produced by the coexistence of different types ; melody thus corres- 

 ponding to pure colour, harmony to mixed colour, and unmusical noise to white 



