162 Mr. R. W. Wood on Equilibrium-Figures 



the first surface of one or of a pair of prisms, which should be 

 placed with their edges vertical, and may stand close to the 

 heliostat. The light emerges from the prisms as a divergent 

 beam producing a horizontal spectrum, and one which, if the 

 microscope is set up at a distance of four or five yards, will 

 probably be about two feet long and not too bright in the 

 violet or indigo. If too bright, remove the microscope further 

 from the prisms, or use unsilvered glass instead of the mirror 

 of the microscope ; if not bright enough, bring it nearer. It 

 is convenient to place the prisms on a sole-plate supported by 

 three screws, which make it easy to slope the prisms and thus 

 raise or lower the spectrum so as to cause it to fall on the 

 mirror of the microscope. The condenser will then form, by 

 the light of each wave-length, an image of the sun of convenient 

 size and coincident with the objects in the field of view. 



These arrangements are extremely simple, and furnish in 

 the microscope a uniform field of monochromatic light of 

 exquisite beauty and efficiency; and of any colour that may 

 be desired by simply shifting the.microscope sideways through 

 the spectrum. 



I am, Gentlemen, 



Yours faithfully, 



G-. Johnstone Stone y. 



8 Upper Hornsey Rise, N. 

 June 6th, 1898. 



IX. Equilibrium- Figures formed by Floating Magnets. 

 ByU. W.Wood*. 



TN attempting to repeat before classes Mayer's well-known 

 JL experiment with the floating magnets, many have doubtless 

 been troubled with the lack of perfect symmetry of the figures 

 that arises from the unequal magnetization of the needles 

 and other minor causes. This is particularly the case when 

 more than six or eight needles are used. As the experiment 

 proved so suggestive to Lord Kelvin in its relation to the 

 kinetic equilibrium of columnar vortices, and is of such use in 

 illustrating the equilibrium of molecules mutually repellant, 

 but drawn towards a centre by an outside force, I believe it 

 worth while to draw attention to an improvement on the 

 original form of the experiment, which I find gives perfectly 

 sym metrical figures even when twenty or thirty particles are 

 employed. The apparatus that I have used consists of a 

 large vertical electromagnet with a shallow glass dish partly 

 filled with mercury immediately above, and at a distance of a 

 few centimetres from the pole. Onto the surface of the 

 * Communicated bv the Author. 



