= Coblentz's cavity-radiation spectrum measurement
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<William Weber Coblentz>'s primary report describes an electrically heated porcelain cavity wound with platinum ribbon. Mirrors imaged its central radiating diaphragm onto a $0.6\,\mathrm{mm}$ <optical slit>; a fluorite <optical prism> dispersed the beam onto a vacuum <bolometer>. <Galvanometer> deflections read the detector response. A resistance standard checked electrical sensitivity, while repeated readings checked stability. <Sodium> lines fixed the angular zero; a <refractive index> curve supplied <wavelengths> and slit-bandwidth corrections. <Thermocouples> were standardized at freezing points of zinc, antimony and copper. Drying the optical path reduced atmospheric absorption; prism scattering and <temperature> calibration remained limitations. These were Coblentz's measurements. His extraction of radiation constants assumed the <Planck law>, so it was not a model-independent proof of the spectral formula.<Constants of Spectral Radiation of a Uniformly Heated Inclosure or So-Called Black Body, I by William Weber Coblentz>[W. W. Coblentz, Constants of spectral radiation of a uniformly heated inclosure or so-called black body, I: printed pp.15–27, 35, 42–47 and 73–77; apparatus, sensitivity checks, temperature scale, wavelength calibration and model-dependent constants]{ref}
The inference to test is whether corrected spectra taken at independently determined <temperatures> have their $B_\lambda$ maxima at a constant product $\lambda_{\max}T$. Comparing raw <galvanometer> deflections at different <optical prism> settings would also compare different <wavelength> widths and optical losses. The response integral in <blackbody peak and detector response> explains why those corrections belong to the physical inference rather than merely to presentation.
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