74 PLANT PHYSIOLOGY AND ECOLOGY 



stop-watch is especially valuable for this purpose, but a watch 

 of the usual type is satisfactory. After a little practice, it is a 

 simple task to hold the watch with the one hand against the 

 photometer in the other, in such fashion that the slide may be 

 moved without taking one's eyes from the second-hand. Imme- 

 diately after each exposure, the disk should be turned to the 

 next line: this should be made an absolute rule. Except in the 

 case of readings made for special purposes, the instrument is 

 held with the edge toward the south, with the opening upper- 

 most in the usual position of the leaf. 



Standards and ordinary readings are made in practically the 

 same manner. There are various kinds of standards, but the 

 most satisfactory for ordinary purposes is a temporary or proof 

 standard bearing a series of exposures. This is obtained by 

 exposing a strip to full sunshine at noon upon a clear day. Expo- 

 sures are made for 1, 2, 3, 4, and 5 seconds successively, the great- 

 est care being taken to make the time of exposure exact. Fre- 

 quently it is advisable to make a second series to serve as a check 

 upon the first. After it is completed the standard is removed 

 and placed in a light-tight box, such as is used for photographic 

 plates. It should be used only in gaslight or in ruby light. The 

 former permits much more accurate comparison. When kept 

 in a cool place and handled carefully, the standard and the solio 

 strips may be preserved for several months without appreciable 

 change. Ordinarily, a single standard is sufficient for an entire 

 growing period, though it is sometimes desirable to make a new 

 one from time to time to serve as a check upon the original. 



91. Making readings. The best practice in making readings 

 is to secure a decided tint that falls between the extremes of the 

 standard. It is practically impossible to obtain a sunshine equiv- 

 alent for very faint tints, and equally difficult to match the very 

 deep ones obtained by long exposure. The most satisfactory 

 method consequently is to expose until a good tint is secured, 

 but one that is not stronger than the 5-second tint of the standard. 

 In deep shade, this often requires a long time, and in such places 

 it is usually more satisfactory to stop the exposure with a lighter 

 tint, approximating the 1- or 2-second exposure of the standard. 

 In taking readings, the date, time of day, station, number of 

 instrument and of exposure, and the length of the latter in 

 seconds, must be carefully recorded. As a rule, readings are 



