160 CARNEGIE INSTITUTION OF AVASHINGTON. 



This was the situation which the Laboratory confronted in 1917, 

 and for which it was required to find a solution in the shortest possible 

 time. The success attained is now a matter of record in previous 

 annual reports of this department and need not be adverted to here ; 

 but our task was not complete with the delivery to the Government 

 of the glass which it needed for war purposes. It remains for us to 

 safeguard the nation, so far as it may lie in our power to do so, against 

 the consequences of another such danger in case it should arise. It 

 has accordingly appeared to be our plain duty to tear aside the mask 

 of secrecy which has hitherto surrounded optical-glass manufacture 

 and, to the extent that our experience has made it possible, to leave 

 such a record that our successors may have the information necessary 

 to make these glasses at some future time if they are needed. Whether 

 this demand may find its origin in peace-time developments — that is, 

 in the requirements of research, of engineering, and of commerce and 

 for instruments of precision — or in the need for fire-control instruments 

 for national protection, need not concern us now. 



In pursuit of this plan, about twenty papers were prepared and pub- 

 lished during the year 1919 (see the report of the Acting Director of 

 the Geophysical Laboratory for 1919, Year Book 18, p. 153), containing 

 the record of the experience of the members of the Laboratory staff 

 with the various processes of optical-glass making. During the year 

 just passed ten more were issued (of which brief reviews will be found 

 in the following pages) ; that is, about one-third of the total published 

 work of the Laboratory for the year was again given up to the same 

 purpose, and still other papers are in preparation. Indeed, it is the 

 purpose of the Laboratory to leave as complete a detailed record of 

 these activities as practicable. When these publications are com- 

 pleted and eventually issued in permanent form, the connection of the 

 Laboratory with optical-glass development as a manufacturing process 

 will cease. No attempt was made to reproduce all the known forms of 

 optical glass, nor to extend the bounds of existing knowledge of glass- 

 making in any particular direction, but enough was accomplished to 

 supply the nation in its time of greatest need and to insure it against 

 another such crisis in the near future. Beyond this it was not deemed 

 to be our duty to go, if indeed the limited time at our disposal had 

 permitted us to do so. 



Quite a new method of studying the intimate structure of matter 

 became available in 1913 through the discovery that X-rays suffer a 

 change in direction — a diffraction— when they are passed through 

 crystals. For instance, if a narrow pencil of the rays passes through a 

 thin section of a single crystal, a diffraction pattern will be obtained 

 which is characteristic of the particular crystal and is presumably con- 

 ditioned by the arrangement of the atoms of which it is composed. 

 Upon this observation as a basis, the British physicists, W. H. and W. 

 L. Bragg, were enabled to place the atoms in a few simple crystals. 



