26 



SCIENCE 



[N. S. Vol. XXXIX. No. 992 



nomena found in the resonance spectra of 

 iodine vapor — phenomena discovered by the 

 author — ought probably to rank highest. The 

 fundamental fact in regard to resonance 

 spectra, namely, that a vapor when illumi- 

 nated by monochromatic light which comes 

 from a single direction will become self- 

 luminous and will reemit light of the same 

 wave-length in all directions, would appear 

 to put this phenomenon into a category en- 

 tirely different from that of ordinary fluores- 

 cence and to make it extremely worthy of in- 

 vestigation. 



The first paper deals with the advantages 

 which came from the use of iodine vapor in 

 place of sodium vapor, and with the structure 

 of the absorption spectrum of iodine, which 

 with Wood's forty-foot spectrograph shows no 

 less than seven lines within the space occupied 

 by the green mercury line alone. Here also is 

 described the curious and unexplained effect 

 of mixing helium with the iodine vapor, 

 namely, the transformation of resonance spec- 

 tra into banded spectra. Another unexplained 

 phenomenon in this connection is the fact 

 that when the exciting beam of light is polar- 

 ized this polarization is passed on to the lines 

 of the resonance spectra. 



The second paper describes the 40-foot 

 spectrograph with its now celebrated " pussy- 

 cat " attachment and also deals with the ab- 

 sorption spectra of iodine and the emission 

 spectrum of mercury. Wood estimates the 

 number of absorption lines in iodine vapor at 

 50,000, and recommends it as superior to sun- 

 light for testing large gratings. Important 

 extensions of this work on iodine vapor are 

 reported in the current (November) number 

 of the Philosophical Magazine. 



The third paper deals with the resonance 

 spectrum of another vapor — that of mercury — 

 which is excited only by ultra-violet light of 

 wave-length 2536. Here is related the dis- 

 covery of " secondary " resonance, a remark- 

 able radiation, which comes, not from the 

 vapor directly illuminated by the exciting ray, 

 but from those portions of the vapor which are 

 illuminated only by primary resonance radia- 

 tion. A most interesting feature of this paper 



is the experimental passage from diffuse to 

 regular reflection of resonating vapor, i. e., 

 from volume to surface reflection. Experi- 

 ments show that the resonance radiation 

 " begins to weaken at a pressure of about 2 

 cm. (mercury vapor) and is practically gone 

 at 70 cm." 



Next follows a not very successful attempt 

 to determine the anomalous dispersion of mer- 

 cury vapor in the neighborhood of A 2*536, 

 by use of Puccianti's method. Among the 

 .most interesting results in these papers must 

 be reckoned those in which Professor Wood 

 has determined, by experiment, the connec- 

 tion between transparency of a layer of metal- 

 lic globules and the diameter of those spheres. 

 Using a layer of mercury droplets deposited 

 as " dew " on a quartz plate and examining 

 them for transparency by use of infra-red rays 

 of wave-length 112 fx he finds that these 

 metallic particles have no effect in stopping 

 long heat waves until their diameter begins 

 to exceed one tenth of a wave-length. Very 

 striking also is the fact that such a sheet of 

 small metallic particles — packed together in 

 such a way that they almost touch — seems not 

 at all to interfere with the transparency of 

 those portions of the plate not covered by the 

 metal; the plate remains as transparent for 

 waves of 112 /n as for waves of 1 ju. 



Paper No. 6, dealing with diffraction grat- 

 ings having a controlled groove-form, does not 

 quite reach the level of those which precede it. 

 The next contribution contains some excellent 

 photographic examples of the reflective power 

 of nickel films on glass and numerous details 

 concerning the author's method of depositing 

 these films. 



Paper No. 8 carries the title of " Selective 

 Absorption of Light on the Moon's Surface 

 and Lunar Petrography," the latter half of 

 which impresses one as rather ambitious. 

 " Petrography " is a word which has too defi- 

 nite a meaning to be employed as a description 

 of local differences of color either on the 

 moon or on any other body. The method 

 employed is that of using different ray-filters 

 for photographing the moon with light from 

 three regions in the spectrum; and then com- 

 bining the three negatives into a three-color 



