ASSAULT ON ATOMS COMPTON" 293 



Last summer while spending a brief vacation in northern Mich- 

 igan, I noticed a fuzzy ring, not very hirge, around the moon. Half 

 an hour later the ring was perceptibly smaller, and within an hour 

 we had to come in out of the rain. 



This ring was due to the diffraction of the moonlight by tiny 

 water droplets that were begimiing to form a cloud. The size of 

 the ring depends upon the size of the water drops — if the drops are 

 small, the ring is big, and vice versa. So when the ring grew smaller 

 it meant that the drops were growing larger. Soon they would fall 

 as rain. 



Our method of studying atoms is very similar to this method of 

 finding out the size of the droplets in a cloud. Instead of the moon 

 we use an X-ray tube, and in place of the cloud of water droplets 

 we use the atoms in air or helium. For the wave length of the X 

 rays bears about the same ratio to the size of a helium atom that 

 a light wave bears to a droplet of water in a fog. The helium atoms 

 spread the X rays out into a halo. This halo, now of X rays scat- 

 tered by the helium atoms, corresponds precisely to the ring around 

 the moon diffracted by the cloud droplets. Likewise here, from the 

 diameter of this halo, we can estimate the size of the helium atom. 

 We can also tell pretty much what it looks like, just as if the atom 

 were under the microscope. 



Plate 2, Figure 3, shows how the helium atom would look if we 

 were to see it with an X-ray microscope. The picture is drawn 

 carefully from the data we have got from the diffraction halos. 

 Of course, it is highly magnified, about a thousand million times. 

 Such a magnification would make a pea appear as big as the earth. 



In the middle of this fuzzy ball somewhere is the nucleus of the 

 helium atom, which has in it the protons. This fuzzy atmosphere is 

 due to the electrons. We noted above that the helium atom has only 

 two electrons in it. You may wonder how with only two electrons 

 the atom can seem so diffuse. Did you ever see the boys on the 

 Fourth of July waving the sparklers to make circles or figures eight? 

 Of course the sparklers weren't in the form of circles ; they appeared 

 that way because they moved so fast. So here, the electrons give this 

 continuous, diffuse appearance to the atom because they are now 

 here and now there, and we have caught a "time exposure " of tlieir 

 average positions. This is, of course, what we would see if we could 

 look at the atom. 



There have been 57 varieties of atomic theories proposed. Lord 

 Kelvin thought the atom was something like a smoke ring; J. J. 

 Thomson said it was a sphere of jelly. Rutherford called it a 

 miniature solar system, while Bohr and Sommerfeld calculated pre- 



