THE problem of the structure of cellulose is one which has not only a fascination for the organic chemist but is also of the greatest importance in many industrial processes. The method of X-ray crystal analysis has been applied to supplement the older chemical methods, and the results of this work were set out by Sir William Bragg in a recent discourse at the Royal Institution which we are glad to be able to print as a supplement to this issue of NATURE. X-rays have gone far to confirm modern views of the structure of cellulose and have succeeded in shedding new light on some aspects of the problem. It has been shown, for example, that cellulose contains large numbers of small crystals which tend so to arrange themselves that they have one direction in common. The outward sign of this selective orientation is the fibrous nature of the material. In the direction of length of the fibre it is found that the atomic pattern repeats itself every 1073 A. The other dimensions of the crystal cell are less certain, but the evidence is consistent with the values of 7.9 A. and 8.35 A. at 84° to each other and perpendicular to the fibre axis. Such a cell contains the substance of four C6H10O5 groups. Along the fibre direction there are chains of glucose rings (five carbons and one oxygen) linked together by oxygen atoms, the pattern repeating itself identically after every two rings, the length of which is 10.3 A. One such chain starts from each corner and one from the centre of the base of the crystal cell.