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The illumination at the point
This sketch shows an arrangement of camera and a special mirror so formed that images in the mirror are seen in the desired double projection as described in the text.
The illuminagraphic camera is here shown standing between two vertical lines of lamps that served as test objects for testing and correcting the figure of the mirror that is the vital part of the instrument.
This photograph illustrates the method of testing the mirror. The images of the lamp filaments should lie on the (extended) calibration lines when the mirror is correctly formed and is in the proper position in relation to the camera.
The camera is here placed on a desk whose illumination is under investigation, and the image in the mirror seen on the base of the ring stand includes all parts of the room above the level of the mirror.
This illuminagraph shows the window, wall, and ceiling in sizes that are strictly proportional to their effectiveness in directing light to the desk shown in the preceding figure.
This photograph was taken to show how external obstructions may influence the light received from a window. Here planimeter measurements show the illumination to be reduced by about one quarter by the buildings seen through the lower part of the window.
This illuminagraph was taken with the instrument set against the face of a drawing board tilted at about 45° and turned at an angle with the window so as to secure the best illumination at a point some eleven feet away.
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