408 



Prof. E. A. S chafer. 



[Mar. 1, 



Tables and curves are given showing the relation between magne- 

 tising force and changes of length, in each metal. 



Bismuth was found to be slightly elongated in strong fields, though 

 no change could be detected with forces of less than about 500. The 

 greatest elongation observed was about 1*5 ten-millionths of length. 



Manganese-steel was almost unaffected. The elongation in a field 

 of 850 was estimated to be about one fifty- millionth of the length. 



Finally, it is shown that the mechanical stress produced in iron by 

 magnetism does not account for more than one-fifth part of the 

 observed magnetic retraction. 



An Appendix to the paper contaius evidence of the high degree of 

 accuracy obtainable by the method of observation employed. In the 

 veiy great majority of the measurements of elongation and retrac- 

 ts n, the probable error was less than one two-and-a-half-millionth 

 part of an inch, or one hundred-thousandth of a millimetre ; and 

 the results of experiments made upon different days (the apparatus 

 having been in the meantime dismantled), or with currents of 

 ascending and of descending strength, were strikingly concordant. 

 This degree of precision is attributed to the perfection of the optical 

 arrangements, which rendered it possible to project the image of a 

 wire with such sharpness, that after reflection from a mirror its 

 position upon a scale 24 feet (732 cm.) distant could be read to a 

 quarter of a scale ' division, each whole division being equal to 

 -^-inch (0 64 mm.). The magnifying power was such that a change 

 of one two-and-a-half-millionth part of an inch (or one hundred- 

 thousandth of a millimetre) in the length of the rod under examination 

 caused the image of the wire to move through about three-quarters of 

 a scale division. More accurately, a scale division corresponds to 

 0-000018 mm. 



The currents used were measured by one of Ayrton and Perry's 

 commutator ammeters, and the accuracy with which the magnetising 

 forces were estimated, though quite sufficient for the purpose of the 

 experiments, does not claim to be very high. 



II. " On Electrical Excitation of the Occipital Lobe and 

 adjacent Parts of the Monkey's Brain." By E. A. SchaFER, 

 F.R.S., Jodrell Professor of Physiology in University 

 College, London. Received February lo, 1888. 



The cortex of the occipital lobe has been explored electrically by 

 Ferrier and by Luciani and Tamburini. In ten experiments upon 

 monkeys Ferrier was unable to obtain any movements on stimulation 

 of this part. Excitation of the angular gyrus produced conjugate 

 de\intion of both eyes to the opposite side, with sometimes an up- 



