=78. Filling the photometer.= The photographic paper called “solio” which is made by the Eastman Kodak Company, Rochester, N. Y., has proved to be much the best for photometric readings. The most convenient size is that of the 8 × 10 inch sheet, which can be obtained at any supply house in packages of a dozen sheets for 60 cents. New “emulsions,” i. e., new lots of paper, are received by the dealers every week, but each emulsion can be preserved for three to six months without harm if kept in a cool, light-tight place. Furthermore, all emulsions are made in exactly the same way, and it has been impossible to detect any difference in them. To fill the photometer, a strip exactly 6 mm. wide is cut lengthwise from the 8 × 10 sheet. This must be done in the dark room, or at night in very weak light. The strip is placed on the wheel, extreme care being taken not to touch the coated surface, and fixed in position by forcing the free ends into the slit of the wheel by a piece of cork 8–9 mm. long. The wheel is replaced in the case, turned until the zero is opposite the index line, and the instrument is ready for use. =79. Making readings.= An exposure is made by moving the slide quickly in such a way as to uncover the entire opening, and the standard if the exposure is to be very short. Care must be taken not to pull the slide entirely out of the groove, as it will be impossible to replace it with sufficient quickness. The time of exposure can be determined by any watch after a little practice. It is somewhat awkward for one person to manage the slide properly when his attention is fixed upon a second hand. This is obviated by having one observer handle the watch and another the photometer, but here the reaction time is a source of considerable error. The most satisfactory method is to use a stop-watch. This can be held in the left hand and started and stopped by the index finger. The photometer is held against it in the right hand in such a way that the two movements of stopping the watch and closing the slide may be made at the same instant. The length of exposure is that necessary to bring the tint of the paper to that of the standard beside it. A second method which is equally advantageous and sometimes preferable does away with the permanent standard in the field and the need for a stop-watch. In this event, the strip is exposed until a medium color is obtained, since very light or very deep prints are harder to match. This is later compared with the multiple standard. In both cases, the date, time of day, station, number of instrument and of exposure, and the length of the latter in seconds are carefully noted. The instrument is held with the edge toward the south at the level to be read, and the opening uppermost in the usual position of the leaf. When special readings are desired, as for isophotic leaves, reflected light, etc., the position is naturally changed to correspond. In practice, it