RECENT ADVANCES IN SCIENCE 21 



or four known planes are observed and a structure is sought 

 which gives the spacings and intensities observed for these 

 planes. In the Hull method a single photograph is taken 

 containing reflections from a large number of unknown planes, 

 and a structure is sought whose whole pattern of planes, ar- 

 ranged in the order of decreasing spacing and omitting none, 

 fits the observed pattern. In both cases the method is one 

 of trial and error, namely to try one arrangement after another, 

 beginning with the simplest, until one is found which fits. 

 The paper gives photographs and details for working out 

 the structure in the case of aluminium, iron, silicon, magnesium, 

 sodium, lithium, nickel, graphite, diamond — the results in the 

 case of the last-named substance forming an excellent check 

 for the method, since the structure of the diamond has already 

 been completely determined by the Braggs, and is given in their 

 book on X-Rays and Crystal Structure. When single crystals of 

 sufficient perfection are available, the Bragg method is, of 

 course, the simplest. When, however, perfect order of crys- 

 talline arrangement cannot be obtained, the next simplest 

 condition is random grouping of small crystals, with equipar- 

 tition of reflecting opportunity among all the crystal planes. 

 There are two disadvantages to this ; the opportunity for any 

 one plane to reflect is so small as to necessitate long exposures 

 of several hours, and the linear images due to all the planes 

 appear on the same plate, so that it is impossible without 

 calculation to tell which image belongs to which plane. How- 

 ever, it permits of definite numerical calculation of the position 

 and intensity of each line, and is free from uncertainties due 

 to imperfection and twinning of crystals. These photographs 

 are preliminary ones, taken with rather crude experimental 

 arrangements, but the author hopes that with improved 

 apparatus and more accurate photographic measurements 

 some evidence may be forthcoming not merely as regards the 

 structure of the atoms in a crystal, but also of the structure 

 of the nucleus and electrons within an atom, at all events in 

 the case of the light substances. Prof. Hull states that there 

 is evidence that some electrons are really free in the sense 

 that they belong to no atom, but occupy definite spaces in 

 the lattice, as though they were atoms. 



To the same number of the Phys. Rev., Blake and Duane 

 contribute two papers. In one they determine the value of 



