LIGHT 



63 



leaves dried under pressure. The leaves are grouped as desired on the glass 

 of a printing frame, and covered with a sheet of solio. They are then ex- 

 posed to full sunlight, preferably at meridian, and the prints evaluated by 

 means of the multiple standard. This method is especially useful in the 

 comparison of ecads of one species. These differences due to transmitted 

 light are very graphic, and can easily be preserved by "toning" the print 

 in the usual way. 



Expression of Results 



91. Light records. The actual photographic records obtained by photo- 

 meter and sciagraph can at most be kept but a few months, unless they are 

 "toned" or fixed. "Toning" modifies the color of the exposure materially, 

 and changes its intensity so that it can not be compared with readings not 

 fixed. It would involve a great deal of inconvenience to make all compari- 

 sons by means of toned strips and standard, even if it were not for the fact 

 that it is practically impossible to obtain exactly the same shade in lots 

 toned at different times. The field record, if carefully and neatly made, 

 may well take the place of a permanent one. The form is the following: 



92. Light sums, means, and curves. Owing to the fact that the scia- 

 graph has not yet been used in the field, no endeavor has been made to de- 

 termine the light value for every hour of the day in different habitats. 

 Consequently there has been no attempt to compute light sums and means. 

 Photometer readings have sufficed to interpret the effect of light in the 

 structure of the formation, and of the individual, but they have not been 

 sufficiently frequent for use in ascertaining sums and means. The latter, 

 are much less valuable than the extremes, especially when the relative dura- 

 tion of these is indicated. Means, however, are readily obtained from 

 the continuous records. Light sums are probably impracticable-, as the 

 factor is not one that can be expressed in absolute terms. The various kinds 

 and combinations of light curves are essentially the same as for humidity. 

 The level curve through a series of habitats is the most illuminating, but 

 the day curve of hour variations is of considerable value. The curve of 



