254 report — 1879. 



with a few individuals, but on that sensitive surface, the retina, a vast range of 

 individual images of the same species may be impressed, but as excesses and defects 

 neutralise each other, the mean or average image is most forcibly impressed on the 

 mind, and that image constitutes our ideal. We arrive at the idea of beauty in 

 precisely the same way that we arrive by a series of observations at the true place 

 of a star. But it is not necessary, in order to illustrate the mental process by a 

 mechanical process, that we should photograph human beings. W« may take 

 geometrical forms, such for instance, as the genus ellipse, whose transverse and 

 conjugate axes may vary between the limits of 1 : 1 and 1 : 0. The diameters of 

 the mean ellipse, parallelogram rhombus, and oviform are as 1 : 2, a proportion in 

 these figures which has been a favourite one through the ages. The genus ellipse 

 may be divided into species, any one of which may be experimented with, for 

 instance the species lying within the limits of 1 : 1^ to 1 : H, or of 1 : 1£ to 1 : 2, &c. 

 I propose to photograph the impression which such figures would make through 

 an aperture in a revolving disc. 



FRIDAY, AUGUST 22, 1879. 



The following Eeports and Papers were read : — 



1. Report of the Committee fur Procuring Reports on the Progress of 

 Mathematics and Physics. — See Reports, p. 37. 



2. Report of the Committee on Underground Temperature. 

 See Reports, p. 40. 



Report of the Committee on Atmospheric Electricity at Madeira. 

 See Reports, p. 63. 



4. On the Retardation of Phase of Vibrations transmitted by the Telephone. 

 By Professor Silvanus P. Thompson, B.A., D.Sc. 



It was predicted from theoretical considerations by Dubois-Raymond that a dif- 

 ference of phase, amounting to a quarter of a complete vibration, would be found 

 to exist between the diaphragms of two associated Bell telephones, the receiving 

 telephone being a quarter of a vibration behind the transmitter. A more complete 

 theory, worked out independently by Helmholtz and Weber, gave a somewhat 

 contradictory result, and required only a small difference of phase. Recently Konig, 

 in a series of delicate experiments, effected an optical comparison by the method of 

 Lissajons of the vibrations of a pair of telephones, replacing the vibrating discs by 

 tuning-forks armed with mirrors. The experiment is a delicate one, and is per- 

 formed under conditions not free from objection. The author has proposed the 

 following method of observing. A pair of Bell telephones are suspended by wires 

 of about a metre in length, so as to oscillate as pendulums, to frames so disposed 

 as to avoid the possibility of any mechanical transmission of the vibrations. Below 

 the point of rest of each telephone, and at some little distance from it in the plane 

 of its swinging, is placed a steel magnet. After the lengths of the wires have been 

 so adjusted that the telephones will swing in identical periods, one telephone is set 

 swinging. As it alternately approaches and recedes from the magnet, the induced 



