= William Hyde Wollaston
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\b[William Hyde Wollaston] developed optical methods for measuring the <refractive index> and <optical dispersion> of materials. His 1802 paper described determining a specimen's refractive behavior from the boundary at which <total internal reflection> begins in a contacting <optical prism>.<A Method of examining refractive and dispersive Powers, by prismatic Reflection by William Hyde Wollaston>[pp. 365–366, principle and instrument]{ref}
The apparatus made the boundary into a measurement. A specimen contacted a glass prism, and hinged sighting arms located the transition between transmitted and totally reflected rays. Wollaston described a square-prism arrangement with a <Vernier scale> for reading the required geometric quantity. This contact method avoided having to shape the specimen itself into a prism.<A Method of examining refractive and dispersive Powers, by prismatic Reflection by William Hyde Wollaston>[p. 366, description of Figures 1–2]{ref}
In modern notation, <Snell's law> gives the critical-angle relation $n_s=n_p\sin\theta_c$ for a specimen index $n_s$ below a prism index $n_p$. As an original example, if $n_p=1.60$ and $\theta_c=60^\circ$, then $n_s=1.60\sin60^\circ\approx1.386$. The angle is measured inside the prism from the interface normal. These stipulated numbers illustrate the method rather than reproducing Wollaston's measurements.
Wollaston also compared oils sold under the same name and reported differing refractive values. This gave the optical measurement a practical motivation: testing material consistency with only a small specimen.<A Method of examining refractive and dispersive Powers, by prismatic Reflection by William Hyde Wollaston>[p. 366, clove-oil comparison]{ref}
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