IMMEDIATE pigmentation " is described as a brownish-red or greyish discoloration of human skin proA^oked by heavy doses of long-wave ultraviolet and visible light (320-450 nyi, Hamperl et aL, 1939 ; Wiskemann and Wisser, 1956 ; Wisser, 1955). It deA^elops during the irradiation, in contrast Avitli the normal sunburn pigmentation, which accompanies the erythema some days after an irradiation Avitli small doses of sliort-waA^e ultra-violet light (297 nyi). Tliis immediate pigmentation partly fades within one or two hours after the irradiation but leaves some traces for a rather long time. Its cause is supposed to be the darkening of prenielanin Avhich is already present.Our attempts to reproduce this phenomenon lia\^e lead to the obser\^ation of a different effect Avhicli Avill be explained in this communication.
THE APPARATUS.The light source is a Avater cooled super high-pressure mercury lani}) (SP 500, Philips) Avith a glass cover. A natural size image (15 X 2 mm.) ofthe arc is tlii-owii on to the skin by means of two glass lenses each of 4 cm. diameter and 10 cm. focal length. To isolate the single Avave lengths 366, 405 and 436 lujti as they are emitted by the merciny arc. interference filters (Balzers) are used with measured transmissions of' 23, 38 and 40^/o respectively and with a half peak Avidth of 22, 14 and 10 m//. The light ohtained in this Avay is sufficiently monochromatic to exclude the waA' elengths near 300 nyi, for Avhich the skin has such a great sensitivity. C^ontamination with radiation of these wavelengths miglit obscure the interpretation of the results of irradiation A\ ith the longer wavelengths.The intensity of the image of the arc of the SP 500 at 297 nyi A\ ithout filtration would be about 200 mW/cni.^ Before reaching the skin hoAvever. the radiation has to pass through three layers of glass, each liaA^ing a transmission of less thau 10"'-.The interference filters, M^iich are mounted between the glass lenses, haA^e at 207 m// a transmission of less than 10-^. Therefore, the intensity of 2£)7 nyi on the skin is less than 200 x 10-^mW/cm.-With an irradiation time of 15 minutes, less than 2 x UW mWsec./cm.^ of 21)7 m/i falls on the skin. Our measurements liaA^e indicated that the mean dose of 297 nyi (MED 297) to provoke a minimum erythema on the upper arm is 23 mWsec./cm." Thus, with an irradiation time of 15 minutes at the wave-lengths 365, 405 or 436 m/t, tlic upper arm does not receive more 297 mii than one-thousandth ofthe MED 21)7.The intensity at the centre of the image on the skin at 366, 405 and 43(i nyt has been found to l)e 290, 380 and 450 niW/cm.-(the calibration data ofthe manufacturers of the thermopile, Kipp, Delft, A\ere used). Tlierefore, with an irradiation time of
