346 Mr. J. E. H. Gordon on Electricity and Light. [Feb. 13, 



Behind the head, the segments for free motion cannot be moved by 

 the developing segmental muscles until an intercalary segmentation 

 has taken place ; hence the vertebral segments which come between 

 the " muscle-plates " and spinal nerves. 



The head, eschewing such mobility, has developed an axial box for 

 the brain, and beneath this firm structure, the mobile and distensible 

 mouth and throat are swung. 



III. " On an Extension of the Phenomena discovered by Dr. Kerr 

 and described by him under the title of ' A New Relation 

 between Electricity and Light.'" By J. E. H. Gordon, 

 B.A., Assistant Sec. of the British Association. Communi- 

 cated by Professor TYNDALL, F.R.S. Received February 10, 

 1879. 



In November, 1875. Dr. Kerr announced in the " Philosophical 

 Magazine," that he had discovered a new relation between electricity 

 and light. He showed that when glass is subjected to an intense 

 electrostatic stress, that a strain is produced which causes the glass to 

 act like a crystal upon polarized light. 



On Wednesday, February 5, 1879, 1 was working at this experiment 

 in the Royal Institution, and endeavouring, by means of the electric 

 light, to project the effect on a screen, in preparation for a lecture on 

 the next day. 



In the experiment as described by Dr. Kerr, and which was shown 

 plainly on the screen, on February 6, the light is extinguished by 

 the Nicols, and reappears when the coil is set going. 



In the projection experiment a patch of moderately bright white 

 light, about 3 inches diameter, appeared on the screen when the 

 coil was worked. The images of the points inside the glass were 

 about 1-J inches apart. On Wednesday, however, the electrostatic 

 stress was accidentally allowed to become strong enough to perforate 

 the glass. Immediately before perforation there occurred the effects 

 which are the subject of the present communication. 



First appeared a patch of orange-brown light about 6 or 7 inches 

 diameter. This at once resolved itself into a series of four or five 

 irregular concentric rings dark and orange-brown, the outer one 

 being perhaps 14 inches diameter. In about two seconds more these 

 vanished and were succeeded by a huge black cross about 3 feet across, 

 seen on a faintly luminous ground. The arms of the cross were along 

 the planes of polarization, and therefore (the experiment being arranged 

 according to Dr. Kerr's directions) were at 45° to the line of stress. 



