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To Construct The Camera Obscura

Make a circular hole in the shutter of a window, from whence there is
a prospect of some distance; in this hole place a magnifying glass,
either double or single, whose focus is at the distance of five or six
feet; no light must enter the room but through this glass. At a
distance from it, equal to its focus, place a very white pasteboard,
(what is called a Bristol board, if you can procure one large enough,
will answer extremely well;) this board must be two feet and a half
long, and eighteen or twenty inches high, with a black border round
it: bend the length of it inward to the form of part of a circle,
whose diameter is equal to double the focal distance of the glass. Fix
it on a frame of the same figure, and put it on a moveable foot, that
it may be easily placed at that distance from the glass, where the
objects appear to the greatest perfection. When it is thus placed, all
the objects in front of the window will be painted on the paper in an
inverted position, with the greatest regularity, and in the most
natural colours. If you place a swing looking-glass outside the
window, by turning it more or less, you will have on the paper all the
objects on each side the window.

If, instead of placing the looking-glass outside the window, you place
it in the room above the hole, (which must then be made near the top
of the shutter,) you may have the representation on a paper placed
horizontally on a table, and draw at your leisure all the objects
reflected.

Observe, the best situation is directly north; and the best time of
the day is noon.





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