200 



Professor Silvanus P. Thompson 



[May 8, 



Professor W. Hoitz, was successful, using as a source of electric dis- 

 charge the electrified wind which is given off by a metal point attached 

 to the pole of an influence machine. If in a perfectly dark room such 

 a point is placed opposite and at a few inches from a wooden disc 

 covered with white silk and connected at its back or edges to the 

 other pole of the machine, it will be observed to show a pale lumi- 

 nosity over a circular patch where it is struck by the electric wind. 

 If then the object is brought between the disc and the point a shadow 

 will be observed to be cast upon the white surface. Non-conductors 

 do not cast shadows as well as conductors do. A piece of thin mica 

 scarcely casts a shadow at all until it is moistened. Double shadows 

 can be got by using two disks covered with silk facing one another : 

 any conducting object introduced between them casts a shadow on 



Dry plate.^^^^ 



(sss^^SSS 



Flate of Coppjtr 



Fig. 3. 



both. If such a shadow from an electrified point is cast downward 

 upon a sheet of ebonite or pitch, the parts not shaded are found after- 

 wards to remain electrified, and can be discovered by scattering over 

 them Lichtenberg's mixed powders of red lead and lycopodium, thus 

 perpetuating the shadow. 



But now it is possible to produce electric shadows in another way, 

 photographically, as has been known for some years,* from metal ob- 

 jects such as coins, by simply laying them down upon a photographic 

 dry-plate (a gelatino-bromide plate) and sending an electric spark 

 (from an induction coil) into them. 



Fig. 3 shows the arrangement adopted by the Rev. F. J. Smith, who 

 is kind enough to exhibit in the library to-night some scores of his 



Proceedings Physical Society of London,' vol. xi. p, 353, 1892. 



