74 
IOWA ACADEMY OF SCIENCE Voi.. XXVIII, 1921 
used between A and D is a rotating “sectored-disk.” ^ Disks 
which allow as little as 1/180 of the incident light to pass through 
(2° opening) can be very satisfactorily employed. The disk should 
be placed as near the pyrometer lamp as possible.® If it is lo- 
cated near the lens the definition will be bad because of diffraction, 
and it will be especially bad if the opening of the disk is small and 
parallel to the background image (as the opening passes through 
the optical axis of the pyrometer) if the background is a wire. 
Therefore it is also advisable to have the opening perpendicular to 
the background when the opening is passing through the optical 
axis. The transmission of a sectored disc is the same whether the 
opening is all in one sector or is made up of several smaller sec- 
tors the sum of which is equal to the single sector. The transmis- 
sion is also independent of the speed of rotation of the disc but 
the rotation must be sufficiently fast to prevent a flicker. Flicker 
is not only disagreeable for the observer but it also makes the ob- 
taining of brightness-matches an impossibility. 
Black-body furnaces are not easily set up, so it is often much 
more convenient and satisfactory to obtain a calibrated “Standard- 
Lamp” for which a curve connecting current through lamp fila- 
ment and brightness-temperature of filament is furnished. This 
lamp is to be used in place of the furnace. It is usually a lamp 
having a large filament which requires a large current at a low 
voltage. In practice the current must be very accurately deter- 
mined; this is easily done with a standard resistance and a poten- 
tiometer. If the effective- wavelength at which the standard-lamp 
is calibrated happens to be different from the effective-wavelength 
of the filter G oi the pyrometer which it is desired to calibrate, a 
correction must be made; that is, the brightness-temperatures at 
the given effective-wavelength must be changed over into bright- 
ness-temperatures at the effective- wavelength of the pyrometer fil- 
ter. This can be done by means of the formula 
where and 5*2 are the brightness-temperatures at the effective- 
wavelengths Li and Lz respectively, and Tc is the color-temper- 
ature. If the standard-lamp has a tungsten filament this change 
can be easily made since the color-temperatures with the corre- 
sponding true-temperatures have already been worked out.®, ® 
Sometimes, perhaps generally, in laboratory work, a glass win- 
dow, the wall of a glass tube, or a layer of some transparent me- 
dium is present between the source whose temperature is desired 
