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48 CRABTREE, EATON, AND MUEHLER Vol 42, No. 1 Solution A Silver nitrate 10 grams Sulfuric acid (cone.) 5 cc Or Acetic acid (glacial) 20 cc Water to make 1 liter Solution B Sodium chloride 50 grams Water to make 1 liter Solution C Hypo 50 grams Sodium sulfite 20 grams Water to make 1 liter A sample of the non-image area of a print, at least x / 4 X 1-in. or an unexposed sheet processed with the batch of prints, is immersed in an excess of Solution A for 3 to 5 min, treated in Solution B for 3 to 5 rnin, bathed in Solution C for 3 to 5 min, washed, and dried. The transmission density of the untreated and treated sample is determined by means of a suitable densitometer. A piece of a Wratten blue filter, No. 44, is placed over the eyepiece for these readings which are made most satisfactorily in a dimly lighted room. The difference in the two readings is the transmission den- sity of the silver sulfide produced in the paper and is proportional to the quantity of residual hypo in the print. The equivalent hypo content is then obtained from the standard curve shown in Fig. 1. The silver nitrate reacts with hypo to produce silver sulfide in situ, then the excess silver nitrate is converted to silver chloride which is dissolved by hypo. If an excess of silver nitrate is allowed to remain, exposure to air and light will cause a darkening of the silver sulfide stain, and the re'sultant transmission density will not be a true measure of the hypo present in the sample. However, the excess silver nitrate may be blotted from the test print, the print then dried, and the transmission density measured immediately. If the silver sulfide stains are kept for further reference, it is then necessary to follow the procedure outlined above. The quantitative procedure just described can be simplified by adapting it to a spot-testing technique capable of determining residual hypo in prints quantitatively within the range of approxi- mately 0.05 to 0.50 mg per sq in. A series of stains produced by placing a definite volume of 1 per cent silver nitrate on the back of each print in a series of prints having known hypo contents, may be duplicated by water colors or oil paints, and this series of dye stains