A d etailerl experimental study of color, apparent optic angle, anci absorption spectrum (0.3 to 16,u of A) indicates that there arc basic chemical and structural differences in muscovite micas. Color identification is bascd upon s pectrophotometer measurements. A m ethod for quantitative evaluation of absorption band activity is defined so t hat single and mUltiple band intensities within the sp ectrum of an individual specimen and within spectra of different specimens can be compared. Three basic spectral t ypes exist for the 0.3 to l,u A region which are comprised of various weak lin es and absorption regions lying alon g the base of a deep absorption edge ncar 0.32,u . Of the three spectral types, on e is a ssociated with ruby micas, whilc the other two are associated with dark green anrl light green micas. The activities of numerous lin es and bands arc reported. It is s hown t ha t apparent optic axial angle is related to t he position of t he d eep band edge except for a certain class of ruby sp ecim ens which show anomaloLl s val ues . A d efinitive characteri zation of muscovite s heet is formul ated in t erms of the absorption coefficients at 0.'14, 0.49, and 0.58,u. Direct assoc ia tions arc s hown between certa in lines in thc vis ible A range and certain bands in the n ear infrared , while the activities of a number of bands in thc middle infrared arc correla ted to color.

Present capabilities for precision and accuracy are compared with the needs at the working level. A measurements structure is sketched by which reference standards accuracies are propagated. Uncertainties are prescribed for a number of prevalent measurement methods including precision U-tube manometer, McLeod gauge, volumetric pressure division, and dynamic expansion. The development at the National Bureau of Standards of direct absolute measurements standards of high precision and their use in dynamic environments stabilized to better than one part in 103 and ranging from the medium to the ultrahigh vacuum are described. These methods include precision liquid columns for the upper pressure range, interferometer manometer and modified McLeod gauges for the high vacuum range, and a torsion microbalance or Vane gauge for the ultrahigh vacuum range.

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The hermetic testing of semiconductor devices is a challenging subject area because of the need for leak testing large numbers of sealed packages to very fine leak rates, where the packages are of a wide range of materials and internal volumes. The types of measurement methods to be discussed are those presently in use and are represented in both military and voluntary standards. Four of these methods will be assessed briefly along with the relevant standards as to advantages, disadvantages, range, precision, and agreement. The four methods are bubble, weight gain, helium leak detector, and radioisotope test procedures. Present interlaboratory test efforts that have been undertaken to provide suitable test data for guidance in the drafting of new American Society for Testing and Materials (ASTM) standards will be summarized. Future directions will be indicated.

The described instrum ent allows rapid measurement of the apparent optic angle to an accuracy of 5' of arc for samples as la rge as 2 inches in diam eter. This angle is a property pertinent to the quality of mica. Instrumentation is quite simple but dependent upon the complex phenomena of interference figur es produced by biaxial crystals in polarized light. Magnific:;!,tion is great, disp ersion can be determined, and the uniformity of samples can be observed.