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XV. On a Wave-apparatus for Lecture-Purposes to illustrate 

 Fresnel's Conception of Polarized Light. By 0. J. Wood- 

 ward, B.Sc* 



[Plate II.] 



IN the ordinary apparatus for illustrating a plane wave, a 

 series of cranks or eccentrics are attached to one axle, 

 the successive cranks or eccentrics being turned through small, 

 but equal angles. Attached to the cranks or eccentrics are 

 rods, each of which is terminated by a ball, to represent one 

 of the particles of the wave. On turning the axle each ball 

 rises and falls in succession, producing a plane wave. This 

 apparatus is known as that of Powell. 



The arrangement I am about to describe consists of two 

 such apparatus, with the rods connected together at right 

 angles to each other. The axles of the apparatus are linked 

 by a rod so that they turn together; and, finally, an adjustment 

 allows the cranks to be placed relatively to each other in any 

 desired position. 



The apparatus, divested of supports and accessories, is shown 

 in the figure, drawn in isometric perspective. The cranks 

 6, 5, 4, 3, &c. turn with the axle X. The cranks are 

 placed successively at 30° angular distance, corresponding in 

 position to the figures on a clock-dial ; and as there are twelve 

 cranks, one complete wave is represented by a single turn of 

 the axle. 



On the axle X / is arranged a precisely similar set of cranks. 

 Around each crank is put a metal strap connected with a light 

 wooden rod. These rods, a, b, c, &c, </, b r , d ', &c, are now 

 connected in pairs at their free ends each by a pin ; so that 

 each pair now resembles a pair of compasses with the legs at 

 right angles to each other. Around each pin is loosely wrapped 

 a wire having a bullet-cast at one end ; and to the other end 

 of the wire, which, from the action of the bullet, projects ver- 

 tically above the rods, is attached a pearl bead. When each 

 pair of rods has been thus treated, we have a row of beads 

 representing the setherial molecules composing one wave. 



To the axle X is fastened a metal disk, D, to which is 

 attached near the edge a pin, p, surrounded by the loose 

 handle, H. On the axle X' is a similar disk, but in which 

 eight holes are drilled at 45° from each other. The holes are 

 all tapped, that the pin p' ', surrounded by the handle H / , may 

 screw into any one of them as desired. A rod, B, connects 

 the two pinsp and ^/, so that both axles can be turned together. 



* Communicated by the Physical Society. 

 Phil Mag. S. 5. Vol. 12. No. 73. Aug. 1881. L 



