678 Prof. R. W. Wood on Resonance Spectra of 



ruled by Dr. Anderson on the 15,000 machine, and the 

 photographs which I have taken with it of the iodine absorp- 

 tion spectrum show that it yields its full theoretical resolving 

 power of 300,000 in the fourth order, which is bright enough 

 to yield a fully exposed photograph of the solar spectrum 

 with a very fine slit in three minutes, when used with a lens 

 of 12*5 metres focus. My laboratory at East Hampton is in 

 an old barn of very large size, and as temperature differences 

 and striae due to air-currents can be better aA^oided by having 

 the entire apparatus under cover, I first mounted the apparatus 

 in the building. I soon found, however, that vibrations of 

 the building due to wind, and probably expansion and con- 

 traction of its frame due to the changing position of the sun, 

 made it very difficult to secure satisfactory photographs, 

 though I occasionally obtained one showing full theoretical 

 resolving power, I finally determined to mount the instru- 

 ment entirely independent of the building, supporting the 

 grating and the lens on a cast-iron pier outside of the building, 

 and the slit and plate-holder on a similar pier sunk in the 

 ground just inside the wall of my dark-room. Not wishing 

 to order a lens until I had tried out the instrument under 

 the conditions prevailing at East Hampton (I had some 

 fear of vibrations resulting from the surf on the beach a 

 quarter of a mile distant), I borrowed, through the courtesy 

 of Professor Campbell and the regents of the University of 

 California, a very fine six-inch achromatic lens of 40-foot 

 focus. The pier, which carried this lens and the grating, I 

 improvised from two six-inch water mains which were slightly 

 damaged by last winter's frost, and were to be had for the 

 asking. The joints of these pipes had been ground together, 

 and the two were bolted together as when laid for service. 

 The resulting pipe was sunk to a depth of six feet in the 

 ground, and a triangular brass bracket from my junk heap, 

 which had originally formed a truss on an old fashioned 

 support for a large reflecting telescope, was bolted to a bent 

 piece of wrought iron, which was in turn bolted to the 

 flange on the top of the water-pipe. The cell of the lens 

 was fastened to the brass triangle with adjusting screws in 

 the usual manner. The grating was mounted on a circle 

 taken from a discarded spectrometer, which was turned by a 

 worm gear. A bevel gear, removed from a discarded hand- 

 drill, was fastened to the worm, the small gear-wheel being 

 turned by a long rod, made by fastening four lengths of 

 cheap brass-covered iron curtain rod together. This rod 

 passed through the wall of the barn, terminating in a wheel 

 a little to one side of the plate-holder and slit. When photo- 



■Bi 



