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I. ABSORPTION SPECTRA OF TYROSINE, TRYPTOPHANAND THEIR MIXTURES.MEASUREMENTS of the absorption spectra of the aromatic amino-acids have been frequently reported in the literature but the results are not in good agreement.For purposes evident in Part II it was required to know the absorption curves of tyrosine and tryptophan with accuracy. Their absorption curves were therefore redetermined. The method of Vierordt for estimating spectrophotometrically the concentration of the components of a mixture was also investigated in its application to mixtures of tyrosine and tryptophan.Method. Absorption measurements were made throughout on a Hilger Medium Quartz spectrograph E 316 (the dispersion of which is 18-6 cm. from 200 to 400 m,u) with Spekker photometer, using a condensed spark between tungsten steel electrodes as light source. The spectrograph was fitted by the makers with a wave-length scale which was checked at intervals with the mercury arc. The maximum error of the scale between 230-2 and 333-9 mp, was found to be + and -0-2 m,u. When exposing a plate on the spectrophotometer the wavelength scale was printed at the top and the bottom of the plate. The matching was made by eye as follows. The plate to be matched was placed gelatin face downwards on a piece ofglass with a fine cursor line engraved on it at right angles to the spectral strips. Owing to the small play which must be allowed for the plate in the plate holder the spectra on two plates are not always similarly orientated with respect to the edge of the plate. An adjustor was therefore made by which the plate could be rotated slightly and the cursor line made to cut the wave-length scales at identical readings. The spectra were then viewed through a low-power microscope, the plate being moved so that the point at which each pair of spectra appeared to match lay on the cursor line. The wave-length at which the cursor line cut the scales was then read off. This is a more laborious method than the usual one of placing an ink mark below the match point but was adopted in order that several sets of matchings could be made on the same plate without bias.

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