basic, the saturated dibasic, the hydroxy dibasic, the unsaturated monobasic, the unsaturated dibasic, and the unsaturated tribasic acid series of the aliphatic series; the saturated monobasic, the saturated hydroxy monobasic, the saturated dibasic and the unsaturated monobasic acid series of the aromatic series. The methods used to prepare the neutral salts have proved to be of very wide application. Many of these neutral ammonium salts have been prepared and their properties determined for the first time. Some of the ammonium salts of organic acids described in the literature as neutral have been proved to be not the neutral salts, and the properties given are not those of the neutral salts. The neutral ammonium salts of these acids have been prepared by this method and their correct properties determined. This investigation is being continued with many other organic acids, especially the substituted acids. PRECAUTIONS IN THE USE OF THE BUNSEN PHOTOMETER HEAD LINDLEY PYLE Associate Professor of Physics The leading text-books on Physics give an inadequate and, almost invariably, an inaccurate treatment of the theory and use of the Bunsen grease-spot photometer screen. One of the British texts, an excellent work, deals with the subject in some detail but, ignoring absorption effects in the screen as well as effects arising from the usual lack of optical symmetry of the screen surfaces, arrives at an erroneous result. The purpose of this paper is to contribute to photometric literature a simple yet accurate treatment of the Bunsen photometer head. Fig. 1a represents a photometer bench equipped with a greasespot screen and two light sources of intensities I and I' respect ively. Designate the faces of the screen by the numerals 1 and 2. Shift the photometer head until the greased and ungreased parts of the screen appear equally bright to an observer regarding face 1. This does not mean that face 1 of the screen is illuminated by source I to the same extent that face 2 is illuminated by source I'. Designate by m and n the distance of the screen from the sources I and I' respectively. In general I/m2 is not equal to I'/n2. When the grease-spot is invisible on face 1 it will in general be visible on face 2. This balance position merely indicates a certain ratio of the illumination on face 1 to the illumination on face 2, due to the sources I and I' respectively. That is, I/m2÷I'/n2 = R, where the ratio, R, usually differs from unity. This single equation does not yield the numerical value of I/I' since R is at this stage unknown. Now reverse the screen, (Fig. 1b), and again observing face 1, at the same obliquity as before, shift to the position indicated by o and p where the grease-spot disappears exactly as it did in the case of Fig. 1a. It is then certain that the ratio of the illumination on face 1 to the illumination on face 2, due to the sources I and I' respectively, is the same as before, namely, R. That is, I'/p2÷I/02= R. Eliminating R, I/I' = mo/np. A momentary consideration shows that the same end is attained by reversing the positions of the two light sources. It is equivalent to reversing the screen. But in either case observations begun on a chosen face of the screen must be continued on that same face, all observations being made at the same angle of obliquity with regard to the plane of the face,-for the law of variation of brightness with obliquity may be different for the greased and ungreased surfaces. It matters not which face is initially chosen. Had face 2 been chosen for observation (Figs. 1c and 1d), distances q, r and s, t would have marked balance points, and I/I'=qs/rt. In general, m differs numerically from s, and q differs from o. It may prove of interest to examine in more detail the conditions for equality of brightness of the greased and the ungreased regions of the screen. Let the following coefficients be defined for a fixed obliquity of observation. Let B1 and b, represent respectively the brightness of the ungreased and the greased surface of face 1 of the screen when face 1 receives unit illumination from a source in front of it. This brightness is due to diffused reflected light. Let B', and b', represent respectively the brightness of the ungreased and the greased surface of face 1 of the screen when face 2 receives unit illumination from a source in front of face 2. This brightness is due to diffused transmitted light. 2 Let B2 and b2, represent respectively the brightness of the ungreased and the greased surface of face 2 of the screen when face 2 receives unit illumination from a source in front of it. This brightness is due to diffused reflected light. Let B'2 and b', represent respectively the brightness of the ungreased and the greased surface of face 2 of the screen when face 1 receives unit illumination from a source in front of face 1. This brightness is due to diffused transmitted light. 1 1 Thus, b1+b', and B1+B'1 represent respectively the brightness of the greased and the ungreased surface of face 1 when faces 1 and 2 receive, simultaneously, unit illumination from sources in front of them. Similarly, b2+b'1⁄2 and B2+B'2 represent respectively the brightness of the greased and the ungreased surface of face 2 when faces 1 and 2 receive, simultaneously, unit illumination from sources in front of them. 2 2 Assuming that the brightness of a surface is proportional to the illumination thrown upon it, the condition for equality of brightness of greased and ungreased surface of face 1 in Figs. 1a and 1b may be written, (I/m2)b1+(I'/n2)b'ı = (I/m2) B1+(I'/n2)B'ı (A) (B) The condition for equality of brightness of greased and ungreased surface of face 2 in Figs. 1c and 1d may be written, (I'/r2)b2+(I/q2)b′2 = (I′ /r2)B2+(I/q2) B′2 (C) (D) |