1904 - 5 .] Form of Bolometer for Physiological Investigation. 829 
wound to an internal radius of 1 mm., and having a space of 
2 mm. between the coils. He uses arrangements of six flat 
needles suspended by a quartz fibre, needles and mirror weighing 
only from 2 # 4 to 6*5 milligrammes. In my arrangement the 
heavy ring magnet of the Wiedemann does all that is requisite for 
our purposes. A low-resistance galvanometer with light needle 
would answer better to very small increments of temperature, hut 
would possibly be unmanageable if muscles or heart were laid on 
the grating. 
The instrument will be very useful both for teaching purposes 
and for purposes of research, but, owing to its delicacy, will have 
to be used with care. 
If gratings are placed in both bolometer arms, and a half-crown 
from the pocket be placed on one and a penny on the other, the 
spot of light goes to the side of the penny, which is the better 
conductor of heat, but after some time the spot returns to zero, 
and then crosses to the half-crown side, the half-crown retaining 
its heat longer. 
The Du Bois Reymond induction coil cannot be used for 
stimulating through its nerve a muscle placed on a grating, because, 
even when the wires are led down from the ceiling and the 
electrodes placed at right angles to the grating, there is a swing of 
the spot of light at each break, followed by an arrest of swing at 
each make. 
Under certain atmospherical conditions, the apparatus shows 
that the heart of a frog is hotter than its muscles, and the tempera- 
ture of the isolated heart gradually rises as it continues beating. 
The heavy magnet does not show a rise for each beat, but rather a 
gradual ascent with stops. A lighter magnet would probably show 
the effect of each beat. 
With regard to the material to be used for the gratings, I find 
that tinfoil is quite convenient. Since, with the arrangement I 
adopted, a low resistance in the grating is advantageous, theoreti- 
cally silver would be the best material to use, but one is met with 
practical difficulties as regards thickness of the commercial article. 
Silver leaf gives too great resistance, and silver foil is too thick. 
It seems to me that Professor Langley attaches too much 
importance to the material of which his bolometer strips are 
