PHYSICS — WOOD, ZAHM. 233 



films obtained in the usual way. lyOrd Rayleigh explained the remarkable 

 purity of the reflected light by assuming that the reflection took place from 

 a large number of parallel twinned planes, sensibly equidistant, a steepening 

 of the intensity curve resulting, as in the case of the Fabry and Perot inter- 

 ferometer. It has appeared that Stokes's explanation of the occurrence of 

 more than one band in the spectrum by assuming two or more sets of twinned 

 planes was unnecessarilj' complicated, the simpler explanation being to 

 assume a single set of reflecting planes and consider the colors as of differ- 

 ent orders. To test this a study of the position of the bands in the spectrum 

 of the reflected light has been made in the visible and ultra-violet regions, 

 the position of the bands confirming the above hj'pothesis. It was usually 

 found that the first-order color occurred in the infra-red. 



Zahm, Albert F», Catholic University of America, Washington, District of 

 Columbia, Grant No. 272. Determination of resistance of air to moving 

 bodies. (For previous report see Year Book No. 4, p. 257.) $1,000. 



Dr. Zahm. reports that he has extended the atmospheric resistance meas- 

 urements of cylinders down to wires of the smallest gage. He finds that 

 the resistance of rods and thick wires is proportional to the diameter, but 

 that this law does not obtain for fine wires, the resistance factor rapidly 

 increasing as the diameter diminishes. On the other hand, the resistance 

 varies exactly as the square of the velocity for all sizes, even to wires of the 

 finest gage. The experiments were performed with wires that did not sing 

 perceptibly and in a uniform air-stream that could be maintained steady at 

 various chosen speeds. 



Experiments are now under way to determine the lift and drift of arched 

 surfaces at various angles of wind impact, the object being to investigate 

 the shapes most useful for the sustaining surfaces of gliding and flying 

 machines. 



