266 
tion of sound at sea, such as that of a fog 
horn, could be determined with a close ap- 
proximation to accuracy. His last publi- 
cation in acoustics was on the production 
of audible beat tones from two vibrating 
bodies whose frequencies are so high that 
the separate tones are inaudible. This 
paper was read at the Oxford meeting of 
the British Association in 1894. The ex- 
istence of such beat tones has been doubted 
by some. In the present case I was asso- 
ciated with Professor Mayer during some 
of his experiments, and had previously had 
considerable experience in the use of the 
sources of sound which he employed. As 
to the reality of the results he claimed 
there cannot be the least doubt on the part 
of any person possessing ordinarily acute 
powers of hearing, who will take the trouble 
to repeat the experiments. 
The study of the physiology of sensation 
tempts to the investigation of the problems 
of vision in connection with those of audi- 
tion. During the winter and spring of 1893 
Professor Mayer became much interested in 
the phenomena of simultaneous color con- 
trast, of which it may yet be said that no 
satisfactory explanation is provided by any 
existing theory of color vision. It has been 
common to assume that the modification of 
color perception induced by contrast is due 
to unconscious motion of the observer’s 
eyes, to error or fluctuation of judgment, or 
to incipient retinal fatigue. Professor 
Mayer devised a variety of methods of pre- 
senting these phenomena with unexampled 
vividness, and on so large a scale as to en- 
able him to arrive at quantitative state- 
ments of such subjective color by compari- 
son with standard hues with which they 
were matched on the revolving color disk. 
Vivid contrast hues were readily percepti- 
ble when the illumination was secured by 
means of a momentary electric flash, and 
with entire accordance between different 
observers who had been purposely misled 
SCIENCE. 
[N. 8. Vou. VI. No. 138. 
to expect something different from what 
they actually perceived. The hypothesis of 
retinal fatigue, or fluctuation of judgment, 
or ocular motion, is excluded under such 
conditions. This psychological element of 
color contrast led to the development of a 
disk photometer for measurement of the 
brightness of colored surfaces. In the use 
of it a degree of accuracy was attained in 
excess of that usually found possible with 
the Bunsen photometer. 
While still at the Lehigh University Pro- 
fessor Mayer began investigations on elec- 
tricity, on magnetism and on heat, which 
were resumed from time to time during 
subsequent years in the intervals between 
his acoustic researches. His lecture notes 
on physics were published serially in the 
Journal of the Franklin Institute, and, sub- 
sequently, put into book form. He devised 
a zero-method of comparing the strengths 
of electro-magnets and electrical conductiy- 
ities, made many observations on magnetic 
declination, and improved on previously 
known methods of fixing and photograph- 
ing magnetic spectra. He undertook an 
elaborate research on the effect of magnetiza- 
tion in changing the dimensions of iron and 
steel bars, an effect discovered by Page in 
1837, investigated by Joule in 1842, and 
then neglected for thirty years. It was 
about this time that an unusually large 
electro-magnet was constructed for the 
Stevens Institute of Technology, and was 
employed with much effect in public lec- 
tures. One of these, entitled ‘The Earth a 
Great Magnet,’ was given by invitation of 
the Yale Scientific Club and, subsequently, 
published. In connection with it a special 
form of lantern galvanometer was devised for 
vertical projection,which attracted much no- 
tice. Another lecture demonstration which 
a few years afterward attracted general at- 
tention was that of the configurations formed 
by magnets floating vertically and subjected 
to the attraction of a superposed magnet. 
