Apparatus All types of spectrophotometer are designed to permit substantially monochromatic radiant energy to be passed through the test substance in a suitable form and to allow measurement of the fraction of that i transmitted. The spectrophotometer energy comprises an energy source, dispersing device with slits for selecting the wavelength a band, a cell for holding the test substance, a detector of radiant energy, associated amplifiers, and measuring and recording devices. Some instrument are manually operated, while others are equipped for automatic operation Instruments are available for use in the visible region of the spectrum, usually 380 nm to about 700 nm, and in the ultraviolet and visible regions of the spectrum, usually 190 to about 700 nm. nm Both double-beam and single-beam instruments commercially available and are either is suitable. Depending on the type of apparatus used, the results be displayed may on a scale, on a digital counter, or by a recorder or printer. The apparatus should be maintained in proper working condition. The housing of the optical system should minimize any possibility of errors due to stray light; this is particularly relevant in the short-wave region of the spectrum CONTROL OF WAVELENGTHS Verify the wavelength scale using the absorption maxima of Holmium Perchlorate Solution, the line of a hydrogen or deuterium di lamp or the lines of a mercury vapour are shown below. The permitted tolerance is 1 nm for the range 200 to 400 nm and nm for the range 400 to 600 nm