The Tropics and Pigment 361 



The English Tourist and His Umbrella. 



Any of us who' have lain suffocating on the deck of a steamer 

 as it passed through the Suez Canal and have seen the native 

 Arab children racing along the banks begging for '' back- 

 sheesh " must be compelled to admit that the Arab has acquired 

 some immunity which the white man does not possess. The 

 traveller on the steamer enjoys the advantage of hygienic sur- 

 roundings, and is protected by all that science can afford him, 

 except a refrigerator, yet the Arab runs across the burning 

 sand, into which he frequently tumbles, with a happy uncon- 

 sciousness of any discomfort, and sublimely ignorant of the 

 perils of light, heat, and a humid atmosphere. This incident 

 accurately illustrates the relative position of the white and 

 black races in the tropics. Visitors only invade them during the 

 winter months, and then they are armed with every comfort. 

 At the approach of summer the white races fly to the hills or 

 some other clime for safety. The only explanation to the situa- 

 tion is that the white man is both actinic and diathermanic, and 

 feels the necessity of protection, even to the use of an umbrella. 

 The use of the words actinic and diathermanic mean that the 

 white man's tissues permit the penetration of light and heat 

 rays. How far these rays penetrate it is difficult to determine. 

 Translumiination, as already shown, gives evidence of trans- 

 *parency. The light rays are visible, and the presence of actinic 

 energy is revealed by photography. To determine the passage 

 of radiant heat the following experiment may be performed : — 

 Place in the mouth, with the electric lamp, a thermometer, and 

 with the face of the individual near a thermopile, all other 

 sources of heat being eliminated, discover, if possible, the pre- 

 sence of radiant heat. But of the actual resistance of the tissues 

 to sunlight the following simple experiments will give some 

 evidence. Free blood is opaque. Smear a little between two 

 glass slides, then hold it to the sunlight, and it will be seen 

 that the blood obstructs vision, though transmitting a diffuse 

 red glare ; now inferpose the test slide between a condenser, 

 focussing sunlight on the head of a match. It will not ignite, 

 the blood having absorbed the heat rays. While recognising 



