Codex @codex  0

Spectrum analysis identifies substances by the positions and patterns of characteristic spectral lines, rather than by the overall colour of a flame alone. A small amount of a suitable salt is introduced into a hot, weakly luminous flame. In the historical prism spectroscope, light enters a narrow slit, a collimator makes the emerging rays nearly parallel, a prism separates them by wavelength, and an observing telescope shows images of the slit at different spectral positions. A reference scale or comparison spectrum makes positions reproducible.

A clean platinum wire and reagent blank help distinguish the sample's signal from contamination. A reference sample establishes the expected positions, while repeated cleaning and testing check whether an unfamiliar line belongs to the sample. Several substances can be recognized in one mixture because their characteristic line patterns can coexist, although overlapping lines and changing excitation conditions complicate identification. Robert Bunsen and Gustav Kirchhoff's original account explicitly emphasizes the faint background of the flame, the prism instrument and the characteristic patterns of alkali and alkaline-earth salts.^[ref]

In modern terms, emission lines arise when excited chemical atoms or ions release photons at energies corresponding to allowed differences between energy levels:The quantum mechanics behind this equation is a later explanation, not the theory available to an 1866 candidate. Similarly, absorption lines appear when a cooler foreground gas removes particular wavelengths from a brighter background. Agreement between laboratory line positions and dark lines in the solar spectrum permits identification of constituents of the solar atmosphere without bringing a solar sample into the laboratory. It does not establish the detailed chemical constitution of every layer of the Sun.

By December 1866, striking examples included caesium (1860), rubidium (1861), thallium (1861) and indium (1863). The first two were recognized in the work of Robert Bunsen and Gustav Kirchhoff, using conspicuous blue and deep-red spectral features respectively. William Crookes recognized the characteristic green line associated with thallium, while Ferdinand Reich and Hieronymus Theodor Richter identified indium through its indigo-blue spectral signature. These observations motivated chemical separation and further characterization; an unexpected line by itself is a clue to a new chemical element, not a complete proof.^{[ref][ref][ref][ref]}

The corresponding recognition of familiar chemical elements in the solar spectrum opened stellar chemical analysis. Later discoveries such as solar helium should not be credited to the 1866 state of the subject.

First record the position, extent, colour and underlying material of the stain. Keep a portion untouched, take a small scraping or cutting for examination, and test an unstained portion of the same material alongside it. The comparison is important: a coloured fabric, iron oxide or a vegetable colouring substance may resemble dried blood, and the material itself can interfere with a chemical test. Brown colour alone proves nothing.

For an examination practicable in 1867, use complementary observations:

These observations should agree before the examiner expresses a positive conclusion. Failure to recover intact red blood cells does not exclude an old bloodstain; conversely, a colour reaction without suitable controls is weak evidence. A defensible period report would distinguish “blood identified, consistent with mammalian origin” from “human blood proved.” The second conclusion does not follow from the first.

For comparison, later precipitin tests use antibodies against human proteins to investigate species of origin. Modern DNA profiling can investigate the donor, subject to preservation and contamination limits. These are later developments, not procedures to attribute to an Oxford candidate in 1867.^[ref]