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We have studied the off-axis light transmission properties of the bright state in Pi-cell (-cell) devices as a function of the white state director configuration that is determined by the applied white state voltage and the pretilt angle of the device. Results show that below certain values of the voltage or pretilt angle, the off-axis light transmission properties are insensitive to these parameters and can be described by a previously considered simple model. However, above a critical pretilt angle or white state voltage, the light transmittance is a much stronger function of the incident angle of light. To understand the facts, we develop a model that explains this result and provides a description of the basic issues affecting the optics of these types of devices.

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A method is presented for the determination of small amounts of nickel in ores containing both iron and cobalt. The ore is first treated with dilute hydrochloric acid and gently warmed, so that a large proportion of the sulphur is eliminated as hydrogen sulphide. After complete solution of the sample with perchloric acid, the metals of group 2 are precipitated with hydrogen sulphide and filtered off with the silica. The filtrate is made 8 N with respect to hydrochloric acid and passed through a strong-base anionexchange column; the nickel in the effluent is determined colorimetrically by the hypobromitedimethylglyoxime method. The procedure has been applied to sulphide and oxide copper ores and their concentrates with satisfactory results.