48 BULLETIN 1059. U. S. DEPARTMENT OE AGRICULTURE. 
seedlings later being thinned to uniform density. Unless it is de- 
sired to determine the effect of light on germination, this process 
should be concluded for all pans, under uniform conditions, before 
they are placed in the solarium. The main operations may be 
started just as soon as the seedlings are established. Otherwise 
great care must be used to develop the seedlings similarly in all pans. 
Having reached the proper stage, place 10 pans hi each of the 
three compartments: say. in two north-and-south rows. The three 
compartments are to be maintained at different temperatures: say. 
at mean temperatures of 50°, 60 c . and 70 : F.. with more or less diur- 
nal oscillation in each. In each compartment one row of pans is to 
be given sufficient water to maintain its soil moisture at. say. twice the 
wilting coefficient, while the other row will be maintained at four 
times the wilting coefficient. The condition of the pans may at any 
time be determined by weighing and the water supply regulated ac- 
cordingly. In any row of five pans five different light intensities 
may be maintained. One pan in each row should doubtless be al- 
lowed full sunlight, another should be cut 20 per cent, a third 40 per 
cent. etc. The amounts may be governed by previously gained em- 
pirical knowledge of the requirements, so that the full range of 
light values will not have to be covered in any case. The shading 
of each pan separately may be arranged by using covers of punched 
screen, in which the areas of the openings correspond to the propor- 
tion or full light which it is desired to admit. It should be borne 
in mind at the outset that the glass of the solarium considerably 
reduces the light intensity, particularly in the infra-red rays. The 
quality of this light should be compared with that of direct sunlight 
and means should also be devised for measuring the light intensity 
under each screen. 
The proper heating of the various compartments will prove the 
most serious obstacle in most cases. This, of course, will have to be 
accomplished by artificial means and should be done by introducing 
warm air into the compartments from an outside source in such a 
manner as to maintain the desired air temperatures without directly 
heating the pans. The air thermometer and thermostat should be 
suspended at a mean elevation and protected from insolation. 
The positions of pans in each compartment should be frequently 
changed so that none will profit more than others by localized heat 
and light optima which are certain to exist. 
The final effect of light and also the effect of other factors with 
light is to be determined by the accretion of dry matter in the seed- 
lings of each pan and species. In order that this may be expressed 
in net quantities for the time of treatment, some of the sedlings 
weeded out at the beginning of the test should be dried and weighed. 
It is evident that this general plan might be followed in tests to 
show the effect of different kinds of light on growth, using mono- 
chromatic screens as covers for the pans instead of the punched me- 
tallic screens, or supplying different compartments of a dark chamber 
with various kinds of artificial light. 
4. The fourth method of determining tolerance or light require- 
ments is similar to that just described, but depends on the measure- 
ment of liffht intensities as they are encountered in the field, and the 
