Abstract
The recently developed methods using the quartz spectrograph in the quantitative determination of inorganic elements in biological fluids require some sort of an instrument for the accurate evaluation of the lines on the photographic plate.
Standard densitometers for this purpose are available but their cost is a handicap to laboratories where spectrographic analysis is being conducted on a comparatively small scale. We have tried to overcome this handicap by constructing a simple device from apparatus available in the laboratories.
Inasmuch as only a portion of the photographic deposit is measured it is called a micro densitometer. It is composed of a binocular microscope which serves as the optical system, a microscope lamp with a source of constant current, a sensitive photo electric cell, and a sensitive deflecting galvanometer.
The photo electric cell of the Voltaic type is mounted on one ocular of the microscope while the other ocular is used for vision of the field under examination. In order to measure only the light that is transmitted through the photographic deposit, the high power or 4 mm lens is used and also a metal field restrictor is placed in the ocular below the photronic cell. In the other ocular of the microscope a similar restrictor is placed which differs from its mate only in having a larger opening on each side of the slit so that a considerable portion of the field may be visible for orientation purposes.
A six volt automobile headlight bulb mounted in a microscope lamp and lighted from a storage battery is used for illumination. The microscope lamp is fitted tightly to the microscope and the light is uniformly diffused by placing a ground glass lens between the lamp and the condenser of the microscope.
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