561 
of Edinburgh , Session 1877 - 78 . 
opposite the proximal telephone ; a fine glass rod or filament, 3 
inches in length, was cemented to the centre of the disk of the 
distal telephone ; to the other end of this rod two common micro- 
scopical covering glasses, having between them a drop of blood, 
were also cemented so as to be in a horizontal position. The drop 
of blood thus fixed to the telephone disk was then brought under a 
Hartnack’s ^th inch, magnifying about 300 diameters, and the 
coloured blood corpuscles were brought into accurate focus. A key 
was placed near the distal telephone, by which the current might 
be transmitted or interrupted at pleasure. On opening the key, 
the circular-coloured corpuscles at once assumed an oval form, and 
were put somewhat out of focus, plainly the result of lateral move- 
ment. Thus, by an application of Lissajoux’s method of observing 
vibrations optically, the movements of the disk could be seen. I 
found that the amplitude of the movements of the vibrating fork 
(moving 60 vibrations per second), near the proximal telephone, 
was about -g-th of an inch ; the movements of the corpuscles were 
about half their diameter, or about g^g th of an inch. On stopping 
the current by means of the key, the movements almost immedi- 
ately ceased. Again, on taking away the glass rod from the disk, 
and applying the ear to the disk, the low booming sound of the 
fork could be heard. It is, therefore, evident that these sounds 
were produced by movements of the disk, the amplitudes of 
which were about the - 6 oVo th of an inch, an interesting example of 
the sensitiveness of the ear. The minimum limit of excitation of 
the ear has been thus stated : — The faintest sound perceptible is that 
caused by a ball of pith, 1 milligramme in weight, falling 1 milli- 
metre in height upon a glass plate, may be heard at a distance of 
91 millimetres from the ear ( Schafhdutl ). 
The telephone appears to be an instrument which illustrates the 
extreme sensitiveness of the ear, even better than the method of 
Schafhautl just alluded to. The length of the hair cells in the 
cochlea (which are stated to be tolerably stiff and rigid) is about 
the -g^nr^th of an inch. One of them could therefore perform 
an excursion of the growth of an inch. If, as Helmholtz 
states, the mechanical arrangements of the bones of the ear 
are such as to diminish to one-third the excursion of the mem- 
brana tympani, and if no further reduction took place in the internal 
