430 MR. C. T. R. WILSON ON THE CONDENSATION NUCLEI PRODUCED IN 
Observations were now made without any glass screen, the quartz lens being still 
used to concentrate the light. The expansions were made alternately with the 
a})paratus unscreened, and with a screen of black paper in front of the quartz plate. 
The black paper was removed immediately after the expansion, to enable the drops to 
be seen. 
The pressui'e fall being 174 millims., the droj^s were very few when the black paper 
was interposed, while a fog resulted when the expansion was made without the 
screen. 
Similar experiments, made with a glass screen interposed, the exj)ansion being the 
same as before, showed again a very marked difference between the results of 
expansions made with and without the black pajjer screen. 
The difference is, in fact, more marked than that between the showers or fogs 
obtained with and without the glass screen. 
It is 2 )lain, therefore, that sunlight, unlike the light from the other sources tried, 
contains nucleus-producing rays which can penetrate glass. 
The Idack paper and the window-glass screen were now removed, and exjDansions of 
the same amount as l3efore made with a red glass screen interposed. Only a few 
drops were produced. On substituting a screen of blue glass, a fog was obtained 
under the same conditions. These active rays can thus penetrate blue glass, but not 
red glass. 
In connection with the above results it is of interest to notice that Elster and 
Geitel (‘ Wied. Ann.,’ vol. 38, p. 497, 1889) found that the actino-electric effect of 
sunlight was not stopped by window-glass or blue glass (red glass being almost opaque 
to it), while glass is quite opaque to the active rays from a zinc-spark or arc. 
To determine to what extent the unconcentrated lig-ht of the sun was effective in 
producing nuclei, the quartz lens was removed, and expansions again made with and 
without a black paper screen, which was removed immediately after the expansion. 
A glass lens was interposed immediately after the expansion to make the drops 
readily visible. 
With expansions sufficient to give a few drops in the absence of sunlight, compara¬ 
tively dense showers were obtained when the air had been exposed to the rays 
immediately before expansion. 
In Aitken’s experiments on the effect of sunlight the expansion Avas probabl}^ not 
sufficiently great to make condensation take place on the nuclei produced by it in 
moist air. Moreover, nuclei, Avhich require such a large degree of supersaturation of 
Avater vapour before it can condense upon them, haA^e never been found to persist for 
more than a feAv seconds ; Avhile in Aitken’s experiments the exposure to the sunlight 
Avas made at an open AvindoAv and the appai'atus then removed to a dark room before 
the expansion was made. 
Although in these sunlight experiments no nuclei, requiring only slight super¬ 
saturation to make condensation take place on them, have been produced, they do not 
