430 Prof. Challis on the Aberration of Light. 



by means of its reverser, a change of deflection is produced; the 

 same is effected if the polarity of the magnets be changed by the 

 reverser which belongs to them. For a fall description of all 

 these phenomena I must refer the reader to the paper on the 

 nature of the diamagnetic force, already mentioned. 



10. To those acquainted with what has been done of late years 

 in diamagnetism, numerous other experiments will suggest them- 

 selves. The antithesis of two isomorphous crystals, one mag- 

 netic and the other diamagnetic, the general phenomena of 

 magnecrystallic action, and the analogous effects produced by 

 pressure, may all be exhibited. 



By placing one of the helices of the electro-magnet upon the 

 other, a coil of double length is obtained, and two such coila 

 may be formed from the four which we have described. For the 

 additional expense of the iron merely, a single electro-magnet, 

 far more powerful than either of the others, because excited by 

 twice the quantity of coil, may be obtained. 



I think it would be an improvement if the suspensions were 

 independent of the glass case, so as to permit of the entire removal 

 of the latter. The best way of showing the deflection of the bis- 

 muth bar within the central helix to a large audience, is to attach 

 a long, light index to the bar itself, and permit this index to enter 

 a French shade which will protect it sufficiently from currents of 

 air. With this arrangement the motions are strikingly evident, 

 and may be seen by hundreds at once. The instrument above der 

 scribed was constructed by Mr. Becker, of Newman Street, and 

 its cost is about twenty-four pounds. It was not my intention 

 originally to have so much wire round the electro-magnets ; and 

 the effects may also be made manifest with a smaller central coil. 

 I have no doubt that with 8 lbs. of wire round each limb of the 

 electro-magnets, and a central coil weighing 4 lbs., the experi- 

 ments might be exhibited to a large audience with perfect di- 

 stinctness. A sensible diminution of cost would of course 

 accompany this diminution of material and labour. 



LX. Note on the Aberration of Light. 

 By the Rev. Professor Challis, M.A., F.R.S., F.R.A.S* 



THE effect of the aberration of light on the apparent place 

 of any body of the solar system, is taken account of by 

 the astronomical calculator on the following principle : — The 

 apparent direction of the body, as affected by aberration, is its 

 real direction at the instant the light by which it is seen started 

 from it. This rule is enunciated by Dr. Young in vol. ii. of his 



* Communicated by the Author. 



