48 



THE HABITAT 



These may be combined in a series for the comparison of readings made at 

 various heights in the stations. The day or point curve shows the fluctuations 

 during the day of one point, and the station curve the variation at different 

 heights in the sam.e station. The curves of successive days or of different 

 stations may of course be combined on the same sheet for comparison. Level 

 and station curves based upon mean relative humidities are especially 

 valuable. 



75. Records. A field form is obviously unnecessary for the psychrograph. 

 The record sheets constitute both a field and permanent record. The alti- 

 tude and other constant features of the station and the list of species, etc., 

 are entered on the back of the first record sheet, or, better, they are noted 

 in the permanent formation record. For psychrometer readings, whether 

 single or in series, the following record form is employed : 



On page 47 is given a table for the conversion of Centigrade into Fahren- 

 heit temperatures. This may be done mentally by means of the formula 



-LIGHT 



76. Methods. All methods for measuring light intensity, which have been 

 at all satisfactory, are based upon the fact that silver salts blacken in the light. 

 The first photographic method was proposed by Bunsen and Roscoe in 1862; 

 this has been taken up by Wiesner and variously modified. After consider- 

 able experiment by the writer, however, it seemed desirable to abandon all 

 methods which require the use of "normal paper" and "normal black" and to 

 develop a simpler one. As space is lacking for a satisfactory discussion of 

 the Bunsen-Roscoe-Wiesner methods, the reader is referred to the works 

 dted below.^ Simple photometers for making light readings simultaneously 



' BuKSEN, R., AND RoscoE, H. Photometrisch6 Vntersuchungen, Poggendorflf's 

 Anrialen., 117:529. ]862. 



WiESNEK, J. 



Photometrische Untersuchungen auf pflanzenphysiologischen Gebiete. Sitzb. Akad. 

 Wiss. Wien., I, 1893. 11,1695. ' " ' 



