1904-5. | NIELS R. Frns—EN—His LIFE AND WorK. IOI 
keenly anxious to probe its mysteries, and now he has a story to tell, and 
the whole world of science waits to hear, sceptical at first, but convinced 
ere long. 
To enter fully into the nature of the researches and experiments of 
Finsen, and all the discoveries which led up to them, while most interest- 
ing and instructive, would yet demand the presentation of an amount of 
scientific data incompatible with the time at our disposal, and the limits 
of a paper such as this; but, in order that the whole matter of his work 
may be fully comprehended, and yet be confined within reasonable bounds, 
it may be permitted to sum up briefly a few of the main theories, but only 
a few, upon which the experiments of Finsen were based; in a word, 
what was the foundation upon which he was to erect his splendid super- 
structure? 
And, first, omitting all that led up to it, was the theory that light is 
composite in character, consisting of various coloured rays, and that when 
it is broken up into its component colours by suitable apparatus a beam 
of light appears to the eye as a band or strip of red, orange, yellow, green, 
blue, indigo, and violet, each shading off gradually and blending with 
many intervening tints into the succeeding colour; this colour series 
being termed the visible spectrum. 
The rays of light are wave-like, and all the waves are not of the same 
length or height, nor do all vibrate at the same speed. ‘The red waves 
vibrate the most slowly, only about four hundred billion vibrations per 
second, and the rate increases as we ascend the series until we find the 
violet rays vibrating at just double that speed, namely, eight hundred 
billion vibrations per second. The red rays, on the other hand, are the 
longest and least easily diverted from their course, or kept back, hence 
they pass most directly through a prism and appear at the foot of the 
spectrum, while the violet rays are of the shortest wave length, and are 
most easily retarded, and diverted, and being most bent out of their course 
in passing through a prism, appear at the top of the spectrum. 
But these rays, red to violet, constitute merely the visible spectrum, 
for beyond and below the red rays, and vibrating still more slowly, too 
slow to be visible to the human eye, are other rays, termed infra-red, or 
ultra-red; and beyond and above the violet rays are others still, which 
are known collectively as ultra-violet, more rapid, too rapid to be detected 
by the human eye, and shorter and also m®re active than the violet, but, 
unfortunately, yet more easily diverted from their course and less pene- 
trating. 
There are many other differences in these varying rays, but differences 
