Abstract
The work of the Bureau of Standards, in studying photographic materials and processes, has a threefold objective, namely, an inquiry regarding the nature and causes of photographic action and phenomena, the application of photography to scientific research, and the standardization of the methods and procedure for testing photographic materials. The sensitometer described in this paper is a special instrument designed for both research and testing purposes, and has, therefore, a more extended exposure range than is necessary for testing purposes. 
purposes.
It is a sector-wheel machine in Althougn this sensitometer has proven entirely satisfactory for general testing, it is not so desirable for studies of certain problems in sensitometry as a nonintermittent apparatus would be; for example, in the study of the failure of the reciprocity law of exposure time and light intensity, (3) because the large number of interruptions with small intensities of light, would, without question, cause serious errors.
purposes.
The design of a sensitometer must depend considerably upon the special nature of the work for which it is to be used. Of previous designs two by Jones (4) of nonintermittent sensitometers are noteworthy. The exposures in the first are controlled by a solenoid which moves a shutter plate in steps in front of the photographic material.
purposes.
The time intervals of the steps are determined by the spacing of perforations in a strip of motion-picture film driven at a constant rate, through which an electrical contact lever operates.
purposes.
The timing of this machine may be changed by substituting another strip of film having perforations spaced to suit the requirements. This sensitometer is adapted for use with small light intensities only as the shutter can not be moved with sufficient precision for exposures less than one-fourth of a second. The other machine by Jones (5) is of the sector-wheel type in which the exposures take place during one revolution only. The speed of the sector wheel is variable through the enormous range of from 1,280 to 0.000614 r. p. m. All of this range is not available, however, as the light-operating mechanism was found not to function satisfactorily at the higher velocities.
purposes.
Apparently the maximum operating speed is 320 r. p. m. The sector wheel is 30 inches in diameter, has 10 steps in the usual powerof-two ratio, and the largest aperture is 240°. This instrument was designed primarily for a study of the reciprocity law.
purposes.
Jones (6) and Higson (7) describe two machines in which an aperture plate is moved in a linear motion; the first one mentioned has step apertures, the other is a continuous wedge-shaped aperture. Another common design is that by Scheiner (11) which has a large number of apertures, usually from 19 to 23 varying in the ratio of 1 to 1.27. In both of these types the smaller apertures are near the edge of the disk and the large ones near the center. Single apertures larger than 90°a re avoided because of weakening of the wheel, so this form could not be used in a nonintermittent sensitometer, as the last aperture would divide its exposure into two parts. This difficulty is surmounted by Jones (5) and Hardy (8) The radial length of the aperture section is 4% inches. fig. 3 ).
purposes.
The control for this is shown at A, Figure 13. The differential relay was made from two ordinary relays by sawing off half of the bridge and putting the two groups of magnets in a position to act on one armature. In this form it is entirely too sensitive to be of use for the purpose. The sensitivity was decreased by fitting to the armature a rod and weight similar to a pendulum, and a vane was secured to the lower end of this which dips in oil for the purpose of damping. Figure 20 shows the completed instrument.
STANDARD LAMPS
Its position on the sensitometer may be seen in Figure 16 .
STANDARD LAMPS
The machine as shown in Figure 16 is complete, the relays and other instruments which are not visible being mounted under the 

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