iv Supplement to ‘‘ Nature,” 
June 9, 1923 

some other way affected by the thing that is seen. 
This means that the thing itself must be comparable 
in size with the wave-length of light. We could not 
expect to gather from the behaviour of a breaker as 
it rushed up the beach information as to the size and 
form of the individual grains of sand over which it 
had passed. We might expect, however, to be able 
to gather information as to the extent of a reef from 
observation of the degree to which it had stilled the 
waves that traversed it before they reached the shore. 
Now, the diameter of an atom is quite a thousand times 
less than the length of the light-waves which affect 
our eyes. Consequently it is out of the question that 
we should ever see it in the sense that we can see small 
objects even under the microscope. A very simple 
way to realise this point is to consider that the atoms 
form part of the very lenses of the microscope ; and, 
if we tried to increase our power of microscopic vision 
by redesigning the optical arrangement, the lenses 
would have become, so to speak, granular and have 
lost their optical properties long before we were able 
to “see atoms by their aid.” The fact is that light- 
waves are adapted for ordinary seeing, and that by 
the microscope we have stretched their proper range 
some thousands of times. Nothing that we can ever 
do with ordinary light will give us the magnification 
of a hundred million times, which is what we require if 
we are to study the atoms themselves. We want a new 
sort of light of immensely finer quality than ordinary 
light ; and we have been fortunate enough to find this 
in the X-rays. X-rays are simply a form of light the 
wave-length of which is ten thousand times shorter than 
that of the light with which we see in the normal way. 
There is one more point to be made clear before 
we can realise how the combination of X-rays and 
the crystal has opened up a new vista. Although 
the X-ray is so fine in structure that it can really be 
affected by the individual atom, the magnitude of that 
effect is too small to be of any use: it is here that the 
crystal helps us. We remember that there is in the 
crystal a perfectly regular repetition of some simple 
pattern or combination of atoms. When X-rays 
sweep over them, whatever effect one of the units has, 
all its fellows have also; and so on the whole there 
is a combined action big enough in its results to be 
detected by instruments designed for the purpose. 
In somewhat the same way, to take an example, each 
tiny furrow on a piece of mother-of-pearl is of the 
right order of width to have an effect on the light 
which is reflected by the whole piece, but the magni- 
tude of one such effect is not enough to make an im- 
pression on our eyes. However, on the surface of the 
pearl there are many thousands of such furrows very 
like one another and running more or less in the same 


direction, and what one furrow does the others do also. 
It is this combined or multiplied action which so 
affects the light as to give the beautiful play of colour 
associated with mother-of-pearl. 
Now we have all the factors essential to the new 
methods: the X-rays for fineness of vision and the 
crystal for combination in the action of the atoms upon © 
the X-rays. It is not necessary now to go into further 
details ; it is only needful to realise that there is an 
instrument called the X-ray spectrometer in which 
the reaction between the X-rays and the atomic arrange- 
ments enables us to study the form and size and disposi- 
tion or structure of the atomic patterns of the crystal. 
Every crystal is in a way a long avenue down which 
we can look and see at the far end of it the most primi- 
tive groupings of the atoms. The wonder is that we 
should be able to look so far, that the structure of the 
crystal should be so finished and so unvarying from 
first to last that our observation of a crystal big enough 
to handle should tell us no more and no less than the 
properties of the one little unit of pattern. If the 
diamond in a ring were increased to the size of the 
earth the individual carbon atoms would only be about 
as big as tennis balls. Yet so faithful is the information 
which X-rays and crystals give us that we can compare, 
and indeed measure, the distances from atom to atom 
with an error less than 1 part in 1000. This new power, 
which is surely wonderful enough, we naturally apply 
to the further elucidation of the problem which I 
described at the beginning. We try to find out, by 
fresh means, the relations between the properties of 
substances and the nature of the atomic structures 
of which they consist. 
It might be objected that a crystal is something 4 
special and that most bodies do not show the perfect 
crystalline form. The difficulty is apparent, not real. 
In the first place, far more substances are crystalline 
than would be supposed, and actually every substance 
would more naturally develop into a perfect and 
characteristic crystal than into any other form. The 
crystal is the natural condition. Bodies which seem 
to us to present no crystalline appearance at all are 
often aggregates of minute crystals jammed together 
miscellaneously or held like a mush in a semi-liquid 
matrix. Often, again, as in the case of liquids, the 
various atoms and molecules have not had time nor 
peace enough to arrange themselves as they would. 
Even if many of the substances in the behaviour of 
which we are most interested, such as iron and steel, are 
far in form from the perfect crystal, yet we may expect 
to arrive in the end at an understanding of their 
structure by the separate examination of the few 
definite forms of crystal of which, as we know well, 
the whole mixed mass is compounded. 
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