Dr. Howard on Electric Radiation. 59 



calculated from its dimensions after the manner of Hertz, 

 is 100 centim. And this is a sufficient amount longer than 

 the conductor itself for the calculation to be not very inexact. 

 It cannot pretend to accuracy. 



The Resonator, or receiver and detector of radiation (the 

 electric eye, as Sir W. Thomson calls it), was of the simplest 

 possible construction. Two pieces of copper wire (No. 13 

 B. W. Gr.) were cut each to a length of 25 centim. One end of 

 each was rounded off, and to the other end was attached a small 

 rectangular brass scrap or plate at right angles to the wire. 

 These little plates each carried a point ; one of these points 

 was fixed, and the other adjustable by a screw, by means of 

 which the distance between them could be varied. The reso- 

 nator was fastened to a piece of wood a little longer than 

 itself. Its total length, including points and strips, was 53 

 centim., i. e. about half the calculated wave-length of the 

 oscillator. A better mode of expressing it is to say that each 

 half of the resonator is approximately a quarter wave-length, 

 and corresponds to a closed organ-pipe, or to a resonant 

 column of air in a glass jar. 



The lenses were made of common mineral pitch, which was 

 found to insulate quite well enough for the purpose. They 

 were cast in the form of hyperbolic cylinders, bounded by a 

 plane perpendicular to the axes of the principal hyperbolic 

 sections; the eccentricity of the latter was equal to 1*7, and 

 was taken as a fair approximation to the refractive index of 

 pitch for infinitely long waves. A lens of this form should 

 converge a bundle of parallel rays falling normally on its 

 plane surface to a line of foci coinciding with the outer foci of 

 its principal hyperbolic sections ; and, vice versa, rays pro- 

 ceeding from this focal line and falling on the curved surface 

 should emerge from the lens as a bundle of parallel rays. 

 Hence, if the oscillator be placed along the focal line of one 

 lens, the electric rays from it will be sensibly parallel after 

 traversing the lens, and after falling normally on the plane 

 surface of the second lens should converge and meet at its 

 focal line. The lenses were almost equal in size. Their plane 

 surfaces w 7 ere nearly square, being 85 centim. high and about 

 90 centim. broad. The greatest thickness (from vertex of 

 hyperbola to plane surface of lens) is 21 centim. The lenses 

 are each separated into an upper and lower half by means of 

 a thin wooden partition inserted during the casting. It was 

 intended to divide this partition by a saw-cat, and thus allow 

 the lenses (each of which w 7 eighs more than 3 cwt.) to be 

 more easily carried about. So far, however, this has not been 

 done. 



