calibration curve also makes it possible to determine ε . As usual, the best line through a series of independent points makes a better calibration curve than one made by one point and the origin, or by several points obtained by dilution of a single sample. Your analyses cannot be better than your calibration curve. For an analysis, the unknown samples are simply run as usual, and from the observed absorbance values the concentrations in the samples are read from the calibration curve. Spectrophotometer A spectrophotometer makes it possible to measure the fraction ( I/I 0 ) of light of different wavelengths transmitted by a sample; I 0 is the intensity of the light beam incident on the sample and I is the intensity of the beam which emerges from the sample. To accomplish this, the spectrophotometer must provide (a) a source of white light of steady light intensity, (b) a means of selecting a beam of desired wavelengths from the white light, (c) a stable light-sensitive detector, and (d) a convenient means for reproducibly placing samples in the light beam. Most instruments use a simple tungsten filament lamp for the light source in the visible region of the spectrum, with stability of light intensity provided by an electronically controlled circuit. Lamp stability is important in order to avoid oscillations of the meter, or drift in light intensity from one sample to the next. By alternating each sample with the blank it is possible to check on the drift and to make a suitable corrective adjustment if necessary. A H 2 discharge lamp must be used for a continuous spectrum in the ultraviolet. The desired wavelength band is selected from the spectrum by rotating, with a calibrated knob, a reflective diffraction grating or a refracting prism . The grating consists of a glass plate, containing about 15,000 parallel and uniformly grooved lines per inch, which has been rendered reflecting by aluminizing. The prism is a 30° - 60° type which is aluminized on the back side so as to reflect the beam back through the prism in the direction from which it came; this doubling- back trick gives the same degree of refraction as a 60° - 60 ° prism and greatly cuts down on the length of the instrument that would otherwise be required. In either case, the reflected beam is dispersed to produce a continuous spectrum. The detector, which measures the amount of light transmitted, is usually a photoelectric cell in which the electrons that are ejected by the photons falling on an alkali metal surface (usually cesium) are collected by a positively charged wire and passed as an electric current through a meter. The higher the light intensity, the more p per unit time, the greater the number of electrons ejected per unit time, and the higher the meter reading.