CHAPTER IV. 



PART I. EXPERIMENTS BEARING ON THE PROPERTIES OF CORONAS. 



47. Introductory. There are a number of obscure points in the theory of 

 coronas when the particles producing them range in size from about io~ 3 cm. 

 to io~ 4 cm. in diameter. These relate chiefly to the colored central disks and 

 to the color which for very fine particles spreads uniformly over the white source 

 of light. In the latter case the colors are strictly axial and they suggest the in- 

 terferences due to thin plates. At least a tentative explanation along these 

 lines seems available.* Light, moreover, is abundantly reflected by the par- 

 ticles, as may be tested by using a Nicols prism. It seems reasonable, there- 

 fore, to assume that in spite of their small size the light is also transmitted and 

 that the effect is appreciable when the column of fog is long enough in the 

 direction of the impinging light. All of this is in accordance with the condi- 

 tions under which axial colors are produced. If they were regarded as dif- 

 fractions within the geometric shadows of the droplets whose diameter d is 

 decidedly smaller than io~ 3 cm., the axial distance b in front of the droplet 

 corresponding to the color X would be b = d z /n\ nearly, for the fringe of the 

 nth order. Hence, even in case of n= i, b would be much less than 0.2 mm., 

 whereas the axial colors are seen for all values of b; i.e., they do not vary 

 with 6, however large it may be taken. 



The disk colors, however, belong to the phenomenon itself. If the element- 

 ary equation for a single particle were true, i.e., if 



where d is the angle of diffraction for the wave-length X and the diameter of 

 particle d, C the constant given by Airy's series, and s/R the aperture of the 

 corona shown by the goniometer, the disks should invariably be white and red 

 edged, as is the case of relatively large particles and small coronas. Actually, 

 however, the white disk is more and more -evanescent as d is smaller, the color 

 being particularly vivid in case of the green coronas, where the disk is almost 

 quite green. The disk and annuli thus recall the appearance of the rotary 

 polarization of a quartz crystal cut normal to the axis, though of course all 

 polarization is strictly absent in the colored diffraction phenomenon. I have 

 in fact endeavored to identify the colors by the aid of a rotary polariscope, 

 fig. 44, B and A being the polarizer and analyzer, Q' the quartz rouge, FC 

 the fog-chamber at a distance from Q', Q" a quartz column sufficiently long 

 to give a white field. Hence the coronas could be seen directly through Q", 



* Barus, Am. Journal of Sci., xxv, 1908, pp. 224-226. 



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