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 bottorn may be readily removed, and the photographic 

 strip placed in position. LXhe 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. Stoelting Co., 31 W. Randolph street, 

 Chicago, 111. The price is $5. 1 



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. 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 x 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 fxee 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 



