50 THE HABITAT 
the successive lines on the disk, and indicates the number of the exposure. 
The wheel contains twenty-five hollows in which the click works, thus mov- 
ing each exposure just beyond the opening. The metal case is made in 
two parts, so that the bottom may be readily removed, and the photographic 
strip placed in position. The water-photometer is similar except that 
the opening is always covered with a transparent strip and the whole in- 
strument is water-tight. These instruments have been made especially for 
measuring light by the C. H. Sree Co., 31 W. Randolph street, 
Chicago, Ill. The price is $5. 
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 x 10 inch sheet, which can be obtained at any supply house 
in packages of a dozen sheets for 60 cents. New “emulsions,” i. @., 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 x Io 
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 en- 
tirely 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 satis- 
factory 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 
