room with sheets of parchment. If we fasten a piece of parchment before the opening in the window-shutter when the sun shines, it will appear nearly white; by adding a second, a yellowish colour appears, which still increases as more leaves are added, till at last it changes to red.
171.
A similar effect, owing to the state of the crystalline lens in milky cataract, has been already adverted to (131).
172.
Having now, in tracing these phenomena, arrived at the effect of a degree of opacity scarcely capable of transmitting light, we may here mention a singular appearance which was owing to a momentary state of this kind.
A portrait of a celebrated theologian had been painted some years before the circumstance to which we allude, by