A photoelectric adapter is described for making measurements of spectral line intensities in the visible and ultra-violet regions. Two integral multiplier photocell units are mounted in a modified spectrograph plate-holder, and provide outputs proportional to the intensities in two selected wavelengths. The outputs may be compared visually or photographically by oscillographic techniques, or may be measured by an integrator circuit. Details are given of the light-tight construction of the units, the system of rapid and accurate line location, the focus adjustment and the slit mechanism. The performance is treated theoretically by considering the possible resolution, sensitivity and precision of wavelength location. The experimental resolution obtained is 1.5 A at 3 000 A.