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 selagraph 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: | 
v “av rm | 
w) 5 on |e ae oe 
3 g be] F Oo =| Sa|s vo} of8.{Fo/2 
Seoillt (3 He 8 g | 3 2 & | | 28/2 Belesies| segs 
gs C} 68 sg = ial D1 vu 6|o%] Ss |&s ge Vere | a | O are 
A x ne a < a to) BLA} AY |) ola" ar lead 
14/9/04 |12:00 m. [Spruce Milky Way 2600 m.|N.E.£0°] Opulaster | 1 foot |2:10| 160s.| 3s | .019) ie 
“ 12:05 p.m.|Spruce Moss Glen 2500 m.j/Level |Streptopus| “ 2:12) 240 s.] 3s.].012]....)...-]....]..-5 
“ 12:15 P.M.|Brook b’nk| Grotto 2500 m./E. 3° | Filix Surface] 2:13} 360 s.| 3.s.].008]....).... |e... [ese 
92. Light sums, means, and curves. Owing to the fact that the sela- 
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 
