290 BURTON E. LIVINGSTON AND EDITH B. SHREVE 



paper, while the other is much less intense tlian that, but never- 

 theless clearly and unquestionably blue. By means of the 

 composite sHps to be described below, these two permanent 

 color standards are brought to the leaf surface simultaneously 

 with the hygrometric shp itself, and closely alongside it. When 

 the slip is dry it is more intensely blue than the more intense 

 one of the color standards. During the process of transfer to 

 the leaf surface and while the glass plate is being appUed this 

 color gradually decreases in intensity, until, some time (a few 

 seconds) after everything is arranged, the color of the hygromet- 

 ric paper just matches in intensity the more intense standard. 



The test is not actually begun until this point of the color 

 change is reached, so that this improvement allows the trans- 

 fer and the placing of the glass plate to consume a little more 

 time than would be thought allowable by the old method. A 

 difference of a very few seconds in the time allowed for these 

 operations is sufficient to determine whether they may be per- 

 formed in a truly efficient manner or whether they must be hur- 

 ried. Undue haste in trying to place the hydrometric slip be- 

 fore it will have altered its moisture content "too much" has 

 doubtless caused many a miscarriage in the beginning of a test. 



Aside from this feature of avoiding undue haste in placing 

 of the slips, this initial color standard greatly decreases the un- 

 certainty regarding their moisture content at the moment when 

 the test is actually begun. By the old method, even if the 

 transfers always consumed exactly the same amount of time, 

 variations in air humidity were still effective to produce an un- 

 known variation in the initial moisture content. Whether a 

 test is performed with high or low air humidity now makes no 

 difference, for the initial color standard is so chosen that even 

 very humid air does not cause the color change to occur so rapidly 

 but that time is amply allowed for all the necessary operations, 

 before the color of the slip comes to match that of the initial 

 standard. Thus most of the initial error is obviated. 



The other permanent color standard is employed to avoid 

 the final error, in a similar way. The color of this standard is 

 a very pale blue, of such quality and intensity that the color 



