NEWS author to give the solvent for crystal lization or optical rotation certainly would be detected by referees or edi tors, the omission of essential informa tion from spectral data often passes unnoticed. Thus, absorption spectra find their way into the literature with no other information than the wave lengths of the maxima. By far the most common omission, however, is the failure to state the solvent, al though solvents often markedly affect ultraviolet spectra. Wave Lengths of Maximal Ab sorption. The position of absorption maxima was found expressed by the measures of wave length (millimicrons and Angstrom units) and the units of frequency (fresnels, wave numbers, or frequencies). Since the most com monly used spectrophotometers will not, in practice, resolve a solution spectrum to better than ±0.5 ταμ, Angstrom units should be chosen only if an instrument capable of such reso lution is employed. The use of wave numbers is advisable in work which is physical rather than chemical in nature. It is necessary, for instance, for the determination of integrated absorption intensities. Since the wave number, whose simple reciprocal rela tion to millimicrons is generally known, and which is often found in infrared work, is available as a unit of fre quency, the use of fresnel or frequency should be discouraged. Nevertheless, any of these units may be found as abscissas, with numbers increasing or decreasing from left to right, although surely most chemists are used to thinking in terms of wave length, and to having the lowest wave length (highest wave number) lie to the left. Since all combinations of terms for describing absorption bands and all possible abbreviations for these terms are used, considerable mental agility is often required to become oriented. Thus a graph of %T vs. fresnels gives a curve which is upside down, backwards, and distorted both vertically and horizontally (S). In the familiar and recommended presen tation of c or log c vs. wave length, the absorption positions and intensities and the general character of the curve are recognizable at a glance. The position of shoulders and inflections may be indicated (and usually is) in the same way as for maxima. The symbol s after the wave length value will be used in Organic Electronic Spectral Data to distinguish them from maxima. Parentheses, footnotes, abbreviations such as infl., sh., letters, asterisks, and other symbols for designating shoulders and inflections are found in the litera ture.
Electron Microscopy With the Hitachi HU-10
The HU-10 offers the chemist and the solid state physicist the ultimate in electron microscopes. Its high resolution and versatility al lows the researcher to pursue investiga tions closer to the ultimate structure of materials. A reflection microscopy accessory per mits examination of opaque materials in their native s t a t e ; heating and cool ing accessories aid in the studies of sur face phenomena at various tempera tures. In addition, an X-ray accessory is available with a magnification range up to 200 X. For further
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ERB & GRAY SCIENTIFIC, Inc. Exclusive Hitachi Exclusive Hitachi distributor distributor for for the the U. U. S.
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