1. The effect of sunlight on sheep artificially photosensitised with eosin
and phylloerythrin was investigated. Glass filters were used to ascertain whether
any of those rays readily absorbed by these dye-stuffs produced skin reactions.
Small square test areas on the back of photosensitised sheep were exposed under
these filters to solar radiation for various lengths of time, up to six hours.
2. A spectral distribution curve was calculated for the solar radiation at
Onderstepoort for an altitude of the sun of 65° and 35° respectively. When this
spectral distribution curve was combined with the absorption spectrum of eosin
and phylloerythrin the resulting curve delimitated those regions of the spectrum
which might be expected to yield skin reactions in the photosensitised sheep.
The results may be summarised as follows:
(a) The skin of closely shorn, non-photosensitised, adult merino sheep,
exposed to solar radiation for several hours showed only a light
erythema which rapidly subsided without oedema formation.
(b) Sheep photosensitised with phylloerythrin (dried leaves of the Lantana
camara plant) and subsequently exposed to sunlight, showed reactions
which increased in severity depending on the duration of exposure.
(c) The reactions of the skin of sheep photosensitised with eosin, were
strictly confined to those parts of the spectrum where eosin strongly
absorbs radiation, namely from wavelength 540-460 mµ, i.e. in the
green and blue part of the spectrum.
(d) The phylloerythrin absorption is spread over a much wider area than
that of eosin. The phylloerythrin shows three regions of absorption
namely a single band from 654 to 626 mµ (orange), a triple band
between 610 and 510 mµ (yellow and green) and high absorption below
460 mµ (blue, violet and ultraviolet). The filter experiments showed
that reactions in the skin of sheep photosensitised with phylloerythrin
were restricted to parts of the spectrum where radiation is absorbed
by phylloerythrin, namely the region between 650 and 380 mµ, which
includes practically all visible light.