JYIanually operated spectrophotometers that are commercially available, and usable in the range of the visible spectrum, vary in price from approximately g400 to $1300. In certain of these instruments the monochromator is a diffraction grating, and the resulting spectral dispersion is linear throughout the wavelength range covered. Other spectrophotometers have quartz or glass prisms as spectral dispersing elements. Considered by some to be a disadvantage is the fact that prismatic dispersion is nonlinear.In recent years interference filters have been developed for use in filter photometers and other optical instruments. Unlike glass or gelatin absorption filters which contain dissolved or suspended coloring agents, most of these newer filters isolate narrow spectral bands through interference of light waves reflected from two partially transparent, parallel, metallic layers separated by a thin spacer layer of transparent material. This principle underlies the operation of the Fabry-Perot interferometer.In Figure 1 the construction of a typical transmissiontype interference filter is shown. Two semitransparent silver films are separated an extremely short distance by a spacer layer of low refractive index material, in this case, magnesium fluoride. In practice, cover glasses are added for protection.Consider now the light ray at the lower left striking the first silver film. Part of this ray is transmitted through both the metallic layer and the magnesium fluoride layer to the second silver film. Here, part

Assuming from a consideration of silicate minerals that aluminum should replace silicon in silicates, the authors investigated the system Naz0-Ca0-SiO~-Alz03. Densities and refractive indices have been determined and molecular refractions have been calculated. Partial molecular refractivities of & 0 3 indicate a coordination number of 4 for aluminum and deviate appreciably from values for compounds in which aluminum has a coordination number of 6. Interionic distances have been calculated for Si-0 and A1-0.As a result of these studies, it is concluded that the aluminum atom isomorphously replaces the silicon atom in the random fetrahedral network. With this replacement, Ca++ can increasingly replace Na+ in the interstices of the open structure.

The density, refractive index, and specific refractivity and their relationship to the composition of glass are discussed. The subject is treated from the standpoint of additive properties based on composition both as to linear and collective or complex functions. Not only are these applied in the rapid checking of constancy of composition in practice but also from the standpoint of possible changes in properties through partial changes of individual components. There is the further possibility of tying up constitution with composition so that a structural analysis may result. The article serves as a review of existing literature and elaborates the possibility of expansion of published data. NOTATIONSsity affects the production cost of articles because the Various authors have adopted different symbols weight per piece depends on it. Density measurement affords a quick check on composition in a plant during routine production. This point will be clarified later for the same expression. ing unified notations will be used:For simplification, the followp,y = weight percentage of X.t P , = mole percentage of X. px = volume percentage of X . w x = weight units of X. = moles of X . ? = density of glass. dx = density factor for X. D-line. nx = refractive index factor for X with respect to sodium D-line. (nc) = refractive index of glass with respect to hydrogen C-line (similar notations for lines of other wave lengths). (nc)x = refractive index factor for X with respect to hydrogen C-line. V = specific volume of glass. vx = specific volume factor for X. k = specific refractivity of glass. kx = specific refractivity factor for X. M = mean molecular weight of glass. Mx = molecular weight of X . V = molecular volume of glass. Vx = molecular volume factor for X . K = molecular refractivity of glass. K s = molecular refractivity factor for S. ?k = refractive index of glass with respect to sodium

IndexIn many series of glasses, refractive index does not vary linearly with composition, and the relationship that does result depends on the type of glass and the composition range studied.Zs~himrner,~T using data after Abbe and Schott, plotted refractive indices against (a) equivalent weights of LizO, ZnO, BaO, and PbO per 100 parts of B203; (b) equivalent weights of Na20, ZnO, BaO, and PbO per 100 parts of alkali-silicate glass; and (c) percentage of B203 in alkali-silicate glass. In no case was a straight line produced, and there was a curve-maximum in (c), The following qualitative conclusions were drawn : (i) The mean refractive indices of Bz03, fused silica, and alkali-silicate glass are raised by the solution of metallic oxides in increments not proportional to the molecular weight of the dissolved oxide; (ii) that variation with concentration is not direct is evident because the optical constants decreased for LizO (in B203), Na20 (in SOz), and ZnO (in B2O3 and alkali-silicate glass) and increased for PbO and BaO; (iii) Bz03 up to 15% in potassium-silicate glass increases refractive index, and further additions reverse the effect.Peddle58 plotted refractive index against moles of a variable constituent. For the soda-lime-silica glasses studied, he was able to calculate that composition necessary to produce a definite value of refractive and dispersion i n d e~.~g His graphs did not indicate a straight-line relation between refractive index and composition.

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