1904-5. ] NIELS R. FinsEN—HIs LIFE AND WORK. 105 
of effect depends also upon the duration of exposure, the amount of pig- 
mentation of the skin, and perhaps the thickness of the epidermis. 
This inflammation is unlike all others of similar duration, in leaving 
a pigmentation of the skin. Unlike that caused by heat, it does not 
develop immediately, and does not attain great intensity until from 
twelve to twenty-four hours after the exposure. It only develops upon 
parts directly exposed to luminous rays, while heat rays may act through 
the clothing. 
Finally, it had been noted that men who worked at a foundry where 
metals were smelted by electricity, suffered very severely from the effects 
of the light upon their skin and eyes; that this was due to the action of 
the ultra-violet rays alone, and not to heat rays was proved by Widmark. 
As far back as 1859, Charcot had expressed the opinion that it was 
the chemical rays and not the heat rays which are active in the cases of 
erythema solare, and that the dermatitis caused by a very strong electric 
light was identical with erythema solare. But it was not until 1889 that 
Widmark gave the scientific proof that such was the case. 
Widmark caused the rays of an electric arc of twelve thousand candle 
power to pass through a layer of water thick enough to absorb the heat 
rays. Under the influence of this light, which contained all the rays ex- 
cept the heat rays, the skin subjected to it developed the characteristic 
inflammation; but when the light was passed through a plate of ordinary 
glass, which excludes the ultra-violet rays, the skin subjected to its in- 
fluence was unaffected. 
Having thus considered the phenomena of light apparent to the 
unaided eye, reported upon by other observers, and having studied the 
particular form of inflammation that was caused by a special irritant, 
Finsen wished to find out what the microscope could teach him about the 
changes produced, and whether the inflammation was a simple one, or 
complex in character. And so, early in 1893 he carried out some experi- 
ments upon tadpoles. Wrapping their bodies in filter paper kept moist 
and cool by cold water, thus keeping them alive and at the same time 
excluding any effects of heat, he placed them upon the stage of the micro- 
scope, exposed to the rays of the sun. After ten to fifteen minutes the 
capillary vessels were dilated, the circulation of the blood slowed, then 
ceased, leucocytes and red corpuscles escaped through the walls of the 
capillaries, as in simple inflammation, and the red corpuscles contracted, 
becoming compressed and more round. This demonstrated that light 
had a direct action upon the smaller blood vessels (capillaries) and 
also upon the blood itself; others had shown that light could cause the 
