430 , PHILOSOPHICAL SOCIETY OF WASHINGTON. 
Mr. E. A. Pace, by invitation, read a paper on The rhythm in 
visual perception. In the attempt to distinguish between physio- 
4 logical and (possible) psychic phenomena the study of phenomena 
on the threshold of sensation may be important. Thus the fail¬ 
ure to perceive a constant, just visible light has been attributed 
to the afferent nerve, to the central organ, and to fatigue of the 
ciliary muscle. This intermittence in vision has been found to 
be parallel to respiration. The speaker’s observations, still in¬ 
complete, on after-images show that the fluctuations are due to 
retinal fatigue. 
The paper was discussed by Mr. Paul with reference to ob¬ 
servations on variable stars, and by Mr. Wead with reference to 
observations by the Fizeau-Cornu method on the velocity of light. 
Mr. J. F. Hayford reported on A test of the transit microme¬ 
ter as a means of eliminating personal equation. In this instru¬ 
ment the observer attempts to keep the wire on the star, and the 
instants when the wire reaches certain positions are electrically 
recorded on the chronograph-sheet. The practical result is that 
for difference-of-longitude work three night observations without 
exchange of observers are equal to ten nights with exchange by 
the old method. [Published in fuller form in U. S. Coast and 
Geodetic Survey Report for 1904, Appendix 8.] 
The paper was discussed by Mr. Paul. 
588th Meeting. October 15, 1904. 
Past-President Dall in the chair. 
Thirty-two persons present. 
Announcement was made of the death of Mr. A. Lindenkohl 
on June 22, and of the death of Mr. F. G. Radelfinger August 
15, 1904. 
Mr. F. W. Clarke spoke on Chemistry at the International 
Congress of Arts and Sciences at St. Louis, outlining the prin¬ 
cipal addresses; the sectional meetings had an attendance of 75 
to 150, the papers were of high grade, and the Congress a decided 
success. The atomic theory was treated as still indispensable. 
[Not published.] 
