Solarisation
A partial tonal reversal produced by briefly exposing a print or negative to light part-way through development.
Measured in
seconds
In depth
Strictly this is the Sabattier effect rather than true solarisation, which is a different phenomenon of gross overexposure. The re-exposure fogs the undeveloped areas while developed silver shields what is beneath it, producing reversed midtones and a characteristic bright line — the Mackie line — along tonal boundaries. Results are notoriously hard to repeat.
Examples in practice
A portrait solarised part-way through development shows a glowing outline traced around the profile.