318 Mr. Mendenhall and Prof. Wood on the Effect of 



copper rod, which dipped for most of its length in liquid air. 

 Glass cones waxed into holes in this cell allowed the 

 (transverse) entrance of the exciting light and the exit of 

 the excited light through a polepiece. These cones were 

 closed with quartz and glass windows respectively, far enough 

 from the cold copper to avoid condensation of moisture. 

 The line is very considerably sharpened at low temperatures, 

 becoming nearly as narrow as the D lines of a flame very 

 poor in sodium. Not the slightest trace of broadening or 

 splitting could be observed, however, with a field of 22,000 

 gauss. An attempt to detect any sharply selective absorption 

 at this same wave-length, both with and without fluorescent 

 excitation, resulted negatively ; but as this was for only one 

 transmission through about 13 mm. of fluorite, it cannot be 

 taken as conclusive against the existence of a corresponding- 

 absorption line. A subsequent experiment in which the 

 path was increased to 36 mm. by multiple reflexions gave 

 also negative results. If the line represents a normal mode 

 of vibration of an electronic system, absorption would cer- 

 tainly be expected, as well as response or resonance to 

 exciting light of the same wave-length. Since the line does 

 not form part of any obvious group or series of lines, one's 

 expectation would be to find absorption and resonance in 

 case a sufficient number of vibrators were available, i. e. a 

 sufficient thickness of crystal. 



The next lines studied were the brilliant red fluorescent 

 pair of the ruby, X 6918, 6932, which are excited by a wide 

 range of visible and ultra-violet wave-lengths, but most 

 strongly by the yellow- green. These lines are much wider 

 than the fluorite line at room-temperatures, but are sharpened 

 more by lowering the temperature, so that in liquid air the 

 difference is not so great, though they never become quite so 

 narrow as the fluorite line at — 185° C. Afield of 45,000 

 volts/cm. produced not the slightest change in the lines. In 

 this case there is a sharp absorption doublet of the same wave- 

 length, but this was also quite uninfluenced by the field. 



In addition to the narrowing of the lines by the low tem- 

 perature there was a shift of 12 A.U. towards the violet, 

 somewhat greater than that noted by Du Bois and Elias. The 

 wave-lengths of these lines, determined from the neon 

 comparison spectrum (neon line 6929*8) are, at — 180°, 6920 

 and 6934. A photograph of the lines under the two con- 

 ditions is reproduced on Plate IV. fig. 3 (all of the photographs 

 are reproduced as negatives). At room-temperature the 

 wave-lengths are 6932 and 6946. The exposures at 23° 



