137.'Actinograph' Slide Rule Exposure Calculator, c. 1892

Signed: 'Hurter & Driffield's Actinograph, Patent No. 5545, 1888. For latitude N 52 30''.

The box has a set of paper scales. First the revolving scale is set to the date and time of day. Next, the speed index is set on the speed of the particular photograph plate on the speed scale, and then the exposure scale is moved until the aperture of the lens in use coincides with the right curve on the revolving scale. The exposure can now be read off for five light conditions: very dull, dull, mean, bright, and very bright.

This, the first in a long line of slide-rule type calculators, was invented by Vero Charles Driffield and Ferdinand Hurter. In 1892 the firm of Marion took over the marketing of this popular instrument.

Length 113 mm, width 67 mm, depth 31 mm.
Inventory no. 89,337.