528 



SCIENCE 



[N. S. Vol. XXV. No. 640 



tionary. This shows that the focal plane 

 of the lens is merely the plane in which the 

 interference fringes formed by light of all 

 incidences coincide, and that the so-called 

 geometrical image is really a superposition 

 of coincident interference patterns; while 

 the usual absence of a sharp image outside 

 the focal plane is due to the more or less 

 uniform illumination caused by the over- 

 lapping of fringe systems formed by light 

 coming from various points in the source. 

 When the grating is illuminated by a 

 parallel beam of white ligbt the effects are 

 similar, except that outside the focal plane 

 the interference fringes are colored. This 

 shows that the focal plane is also the plane 

 of achromatic interference, i. e., the plane 

 in which the fringes due to light of various 

 wave-lengths coincide. 



On the Conductivity of the Air caused hy 

 Certain Compounds during Tempera- 

 ture-Change: Fanny Cook Gates. 

 Investigations on the conductivity of the 

 air, caused by the presence of the sulphate 

 of quinine under certain conditions, indi- 

 cate that it accompanies hydration and 

 dehydration, although phosphorescent ef- 

 fects to which it may be directly due, ap- 

 pear at the same time. The phenomenon 

 is most easily observed during the cooling 

 of the quinine from something over 100° C. 

 to room temperature. 



Heretofore, the only other substance 

 known to produce a similar effect is cin- 

 chonine, which like quinine, hydrates and 

 phosphoresces upon cooling. Both sub- 

 stances are so complex in structure that 

 the exact chemical changes to which they 

 are subject are studied with difficulty. 



During the investigations described in 

 the present paper, a large number of sub- 

 stances were heated to about 150° C, and 

 the conductivity of the surrounding gas 

 was tested while they cooled. Of all the 

 substances tested, none was found to give 



so large an effect as that which results 

 from the sulphate of quinine, but a very 

 definite and marked conductivity was 

 found to accompany the cooling of anthra- 

 cene, and to a less degree that of grape- 

 sugar and aesculin. 



The Transmission of Rontgen Bays 



through Metallic Sheets: J. M. Adams. 



For some years after Rontgen 's dis- 

 covery of the X-rays, it was a matter of 

 doubt whether the absorption of the rays 

 in metallic sheets was accompanied by the 

 development of heat. To investigate this 

 question by means of an instrument differ- 

 ent from those already used for this pur- 

 pose, a radiomicrometer was constructed. 

 The metals forming the thermal couple 

 were constantan and copper. At one of 

 the junctions was placed a small disk of 

 thin platinum to receive the rays, the other 

 junction being shielded from them. The 

 sensitiveness of this radiomicrometer was 

 such that a radiation of 5.6 X 10^' gm. cal. 

 per sec. per sq. em. of its sensitive surface 

 produced a deflection of one scale division. 



Evidence of heat developed in the plati- 

 num by the absorption of Rontgen rays in 

 it was readily obtained. The necessity of 

 making correction for the incomplete ab- 

 sorption of the rays in the platinum of the 

 instrument, together with the well-known 

 fact that the character of Rontgen rays is 

 changed by passage through substances, 

 made it seem desirable to investigate the 

 phenomena of the transmission of the rays 

 through metallic sheets more fully than 

 has been done heretofore. 



The general law of the absorption of the 

 rays in a metal, viz., that each successive 

 equal increment of thickness is less effect- 

 ive as an absorbing medium than the one 

 preceding it, was confirmed by experiments 

 with the radiomicrometer. 



The dependence of the absorbing power 

 of a given metallic sheet upon the intensity 



