This paper by G. Mackinney, published in 1941, focuses on the absorption of light by chlorophyll solutions and the estimation of chlorophyll in plant extracts. The author addresses the issue of artifacts in chlorophyll measurements, particularly the suggestion that chlorophyll as described by Willstätter and Stoll may be an artifact. The study examines the effect of different solvents on the absorption coefficients of chlorophylls $a$ and $b$, noting that these coefficients can vary significantly depending on the solvent used. For example, the absorption coefficients in methanol and ether differ from those in anhydrous acetone. The paper also discusses the importance of using standards that match the solvent and moisture content of the crude extracts for accurate measurements. The author demonstrates that solid chlorophylls $a$ and $b$ can be used to account for the entire absorption curve in the visible spectrum, provided the correct solvent is chosen. The study concludes that the chlorophylls described by Willstätter and Stoll are artifacts only in the sense that they have been isolated from the chloroplastin complex.This paper by G. Mackinney, published in 1941, focuses on the absorption of light by chlorophyll solutions and the estimation of chlorophyll in plant extracts. The author addresses the issue of artifacts in chlorophyll measurements, particularly the suggestion that chlorophyll as described by Willstätter and Stoll may be an artifact. The study examines the effect of different solvents on the absorption coefficients of chlorophylls $a$ and $b$, noting that these coefficients can vary significantly depending on the solvent used. For example, the absorption coefficients in methanol and ether differ from those in anhydrous acetone. The paper also discusses the importance of using standards that match the solvent and moisture content of the crude extracts for accurate measurements. The author demonstrates that solid chlorophylls $a$ and $b$ can be used to account for the entire absorption curve in the visible spectrum, provided the correct solvent is chosen. The study concludes that the chlorophylls described by Willstätter and Stoll are artifacts only in the sense that they have been isolated from the chloroplastin complex.