360 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1933 



The real action of irradiation on the living cell is still unknown. 

 The irradiation has two effects, stimulative and lethal. In general, 

 the stimulative effect is caused by the wave lengths longer than 

 2900 A. The lethal effect is caused by shorter wave lengths, pos- 

 sibly because of the production of a toxic photoproduct which in a 

 small quantity may act as a stimulant to cell division. Finsen 

 (1900) at first thought that the irradiation with which he caused 

 lupus acted as a bactericide. Later there was question as to whether 

 the irradiation did not act as a stimulant to the tissues rather than 

 as deadly to the bacilli. There are two elements in the action of 

 the irradiation, a photochemical one and a biological one. The 

 photochemical effect ends with the production of the dermatitis and 

 the activation of substances in the skin and possibly in the blood, 

 while the biological effect on metabolism, growth, and circulation 

 lasts a longer period (Laurens, 1933). 



An almost magical effect of these invisible rays is their ability to 

 cause fluorescence in ordinarily pale, colorless microscopic organisms. 

 Paramoecia and Oxytrichia (protozoans) fluoresce in all the ultra- 

 violet lines of the quartz mercury arc. Giese and Leighton (1933) 

 report that at 3660 A the fluorescence is of a pale greenish gray 

 shade; at 3350, 3130, 3020, and 2537 A the animals are a pale greenish 

 blue ; at 2804 and 2654 A they have a more whitish light. Darting 

 about in the ultraviolet rays, these habitually drab little creatures 

 are transfigured with gorgeous, brilliant colors for their brief dance 

 of death. 



Bacterial colonies fluoresce according to their land in different 

 colors and different intensities of the ultraviolet, so that Stiibel 

 (1911) has thus proved that ultraviolet can be used as a means of 

 identification of various colonies and races of bacteria. Under the 

 microscope, as shown by the work of the Cancer Research Laboratory 

 (1933) as well, larger micro-organisms are recognized more readily 

 not only by their outlines, which by their own fluorescent light are 

 made visible, but also by the details, such as the nourishment vacu- 

 oles of a paramoecium which can be perceived with perfect dis- 

 tinctness. 



Thus, beyond the band of familiar visible colors that we see in the 

 rainbow lies another band nearly five times as wide of invisible 

 ultraviolet rays which have a profound and mysterious effect on 

 microscopic plant and animal life and which therefore hold promise 

 of great potential benefit to mankind. Physicists, biologists, bac- 

 teriologists, and medical men are uniting to expand our knowledge 

 of these invisible rays, and as that knowledge increases, we shall 

 doubtless be able to record many other beneficial uses of ultraviolet 

 light. 



