The X-point phenomenon, i. e. the intersection of the spectral emissivity (or reflectivity) isotherms at one point at some wavelength A is commonly known in metal optics. This phenomenon was first investigated by Price (1). Later the Xpoint phenomenon was treated in (2 to 7) from several aspects, however, no satisfactory explanation has been given up to the present. In particular it is not clear whether the emissivity isotherms really intersect at one wavelength or in some spectral region, which could not be clarified so far due to insufficient accuracy of the emissivity studies (5).spectral emissivity of Ta for temperatures ranging from 1200 to 2400 OK using a very accurate technique specially developed by the authors and described in (8).The measurements have been carried out in vacuum down to 2 to ~XIO-? Torr. In Fig. 1 (except for the uppermost and the lowest ones), this temperature being displaced from T = 1200 OK with increase of wavelength from 0.76 t c 0.82 pm. Thus, it is concluded that the wavelength Ax, determined as before by the condition a &( 28) /a T = 0 (the intersection point of nearby isotherms) does depend on temperature; as a result, instead of an X-point there is an X-region in which the emissivity isotherms intersect. Fig. 2 shows this region X
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