layer projecting, like steps, beyond the layer above. This step system thus produced, represents a semi-transparent medium, the transparency of which decreases by degrees towards the thicker end. Black figures upon the under side of the lower strip indicate the number of layers arranged above each such figure. The whole cover, with the paper system in it, can be folded down and fastened by the small hook, c, and is provided with a second cover of wood, to protect the glass from injury, when not in use, and to make and close the exposure.
Within the box is a sliding false bottom, D, which is, by means of a steel spring, pressed upwards against the scale described above, when the lid is closed. Upon this false bottom a number of strips of paper are placed, which have been sensitized by immersion in a saturated solution of bichromate of potash of 1 oz. bichromate to 30 oz. water.
In order to get these in place, the bottom of the box is opened, the spring removed, the false bottom taken out (and may be used as a guide to cut the strips of sensitized paper), the strips placed in the box, the false bottom dropped in upon them, the bottom closed, and the photometer is ready for use.
This must be done in the dark-room, and the fingers should be dry. Strips sensitized in this way will keep a month.
It will now be seen that when the apparatus is exposed to light that the sensitized strip changes color in proportion to the amount of light it receives, the most light passing through the spot marked 2, the next 4, the next 6, and so on, and the change will be rapid or slow, according to the intensity of the chemical action of the light. The black figures admit no light through them, and after exposure appear on the sensitized strip as light figures on a dark ground. A strip can only be
