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THE W &T HELLIGE COMPARATOR W ITH FRONT OF CASE REMOVED SHOWING COLOR DISC, RECTANGULAR SAMPLE TURES, ROTTLE W ITH GRADUATED DROPPER, AND DUST-PROOF PRISM. revolving disc form a com pact, readily usable, standard for residual determinations. R evolving the color disc in front o f the sample, the color of standard and sample are m atched and a direct reading in parts per million appears through an opening in the front o f the case. Confusion of color standards is impossible. Rectangular sample tubes eliminate shadow interference. Chlorine concentrations are easily read with accuracy because the 26 m.m. depth o f liquid in the sample tubes produces a color intensity which is easily distinguished even for low residuals. A c­curate readings of colore'd or turbid waters are made by using tw o sample tubes, one containing the ortho-tolidin and the other, an untreated sample. B y this means color and turbidity in the water are auto­m atically com pensated for and a true read­ing of residual is obtained. Where manga­nese or other interference is encountered, addition of ortho-tolidin solution to an unchlorinated sample permits suitable cor­rection, The standard color disc covers normal conditions, having a range from 0.05 p.p.m . to 1.0 p.p.m . arranged in 9 steps. A dustproof eye piece fitted with a glass prism m ay be added at slight extra cost. This prism brings the images of the treated sample and the color standard side by side facilitating quick, accurate reading. W ith the W &T Hellige Com parator, ortho-tolidin tests ate made quickly and accurately.