672 The American Naturalist. [July, 
action of the light on the organisms and not to any indirect action on 
the culture medium, has been brought out pretty clearly by Prof. 
Ward’s labors. That the agar remains unchanged and is still suited to 
the needs of the organism is shown by the fact that some colonies do 
always finally appear on the insolated spots. Their appearance is ex- 
plained by supposing that some spores were covered by others and 
thus partially protected from the action of the light, which might well 
be the case, especially when thick sowings were made. The next step 
was to determine, if possible, whether one part of the spectrum was 
more effective than another, the conclusions of previous experimenters 
being very contradictory. First, a fresh culture was covered by a 
card board in which five circular holes were cut. One of these holes 
was left uncovered, one was covered by ordinary window glass, one 
by a dark blue glass, one by a light blue glass, and finally, one by a 
peculiar brownish-purple glass which absorbed most of the blue and 
violet rays of the spectrum. This plate was then exposed to sunlight 
for some hours and afterwards put into the incubator. In 18 hours 
there were four distinct white spots on the agar corresponding to four 
of the five holes in the card board, and later on that spot correspond- 
ing to the uncovered hole became the most distinct. There was also 
on the agar at first a fainter spot corresponding to the hole covered 
by the brownish-purple glass, but this spot became more and more in- 
distinct and disappeared after the fourth day, enough colonies having 
developed finally to wholly efface it, thus showing that the light strained 
through this glass simply retarded the development of the spores. The 
inference was, therefore, quite strong, that the blue-violet rays largely 
screened out by this glass must be the effective ones. Two-chambered, 
ebonite cells with side walls of glass were then constructed. Into one 
of the cells filtered distilled water was put as a standard for compari- 
son and into the other cell was put solutions of various substances such 
as aesculin, sulphate of copper, bichromate of potash, quinine, fuchs- 
in, etc., which cut out certain rays of the spectrum. Infected films of 
agar were then exposed to the action of sunlight passed through water 
and these solutions. The light which passed through the layer of 
water cleared a spot on the plate every time. The result of passing 
the light through a solution of aesculin, which cuts out most of the 
blue and violet rays, was similar to that obtained by the use of the 
brownish-purple glass, i. e. it did not kill the spores but only retarded 
their germination, the insolated places being nearly obliterated in 111 
hours and entirely soa little later. When sunlight was passed through 
a solution of potassium bichromate the result was still more striking, not 
