402 C. Bar us — Absence of Polarization in Artificial Fogs. 



Art. XXXVII. — Absence of Polarisation hi Artificial Fogs ; 



by C. Barus. 



The astonishing feature of Tyndall's experiment, made in 

 the usual way with motes of mastic suspended in water, is the 

 completeness of the polarization of the light reflected or scat- 

 tered at right angles to the impinging beam. This is particu- 

 larly well shown with the double image prism, in which, for 

 horizontal beams of light with vertical and horizontal vibration 

 beams and line of sight horizontally at right angles to each other, 

 only one of the beams (that for which vibration is vertical) is vis- 

 ible in the turbid water. The other is quite extinguished while 

 lying in a vertical plane either above or below the visible beam. 



In making the same experiment with dense fogs, I was 

 surprised to find an almost entire absence of this discriminating 

 character. Both beams are always in view and about equally 

 intense. These fogs were produced in the fog chamber with 

 phosphorus nuclei and were intensely luminous, with several 

 millions of water particles per cubic centimeter of an average 

 size less than 'OOOl 0111 . Even for fogs so dense and therefore 

 so fine that the light penetrates scarcely 30 cm , both beams were 

 about equally brilliant. On examining the two beams with a 

 Kicol, however, they are found to be almost completely 

 polarized at right angles to each other ; whence it follows that 

 the vibration, which is horizontal outside, has been turned 

 about 90 degrees in a horizontal plane after entering. In 

 other words, whereas it vibrates in a horizontal plane normal 

 to the observer on the outside of the fog chamber, it vibrates 

 in a horizontal plane parallel to the observer in the inside of 

 the fog chamber ; or, while the plane of the two vibrations is 

 normal to the primary beam on the outside, it is parallel to the 

 two beams on the inside of the fog chamber. The beam with 

 vertical vibrations naturally remains unchanged. Finally, if 

 coronas are produced from a polarized source, they are found 

 to be polarized throughout all their colors and quite extin- 

 guished between crossed JSficols. 



As the motes do not produce coronas and will not subside, 

 it is difficult to specify their size. But the marked occurrence 

 of scattering is sufficient evidence for the absence of regular 

 reflection. Virtually at least, the particles are very small. 

 With regard to the fogs, however, even in the case of the very 

 finest particles, the light is still regularly reflected and refracted, 

 and not scattered, as in the first instance, to an appreciable 

 extent. Hence, for a line of vision normal to the primary 

 beam, the direction of vibration may be turned 90 degrees 

 parallel to the plane of the beam and vision, and now vibrates 

 normal to the line of vision, seeing that light is not completely 

 polarized on reflection from a surface of water. 



Brown University, Providence, R. I. 



