Assertion: lens's diameter | /r/CapableOf | observe illuminated patch
Facets:
Source sentences:
text: Make a small round hole in the centre of the cardboard with a piercer, and now remove to a darkened room; apply a candle close to the hole, and observe the illuminated patch visible upon the front combination; the diameter of this circle, carefully measured, is the actual working aperture of the lens in question for the particular stop employed.