Ann Arbor Scientific Association. 21 



their shape there would likely be an excess of crystals having 

 their axes vertical or horizontal, distinct phenomena depending 

 on these positions appear. If the sun is not too far above the 

 horizon, the vertical prisms will throw an excess of light to the 

 right and left, giving rise to lateral mock suns ; the horizontal 

 prisms will, in like manner, produce a mock sun above, and, if 

 the sun's altitude is sufficient, also one below the real sun. These 

 primary parhelia, when brilliant, may be the origin of secondary 

 ones formed in the same manner. 



Another effect of those crystals whose axes are vertical, pro- 

 duced by the light refracted through the terminal edges of 90, 

 is the inverted arc which touches the second halo at its upper 

 point, and having its center at the zenith. The brightness which 

 this arc frequently exhibits, and the order of its colors violet 

 within and red without, the red still being nearest the sun give 

 it a very close resemblance to an inverted rainbow high up in the 

 sky. As this circumzenithal arc and the parhelia on the right 

 and left of the sun are both due to the same position of the pris- 

 matic crystals, whenever one is visible the others generally are 

 also, and this often in the absence of both the primary and sec- 

 ondary halos. 



If the crystals assume a horizontal position, the same angles 

 (90) in like manner produce the two tangent arcs on the lower 

 part of the secondary halo. 



The preceding are all phenomena of refraction. But the 

 light is also reflected from the surfaces of the prisms, as from a 

 mirror. The vertical surfaces thus give rise to the white hori- 

 zontal band passing through the sun, and, if many surfaces are 

 oscillating about a horizontal position, they will occasion a like 

 vertical band through the sun, just as the image of the sun or 

 moon reflected from water not perfectly at rest is lengthened 

 out into a vertical column of light. 



Halos must be distinguished from coronae, which are much 

 smaller, appearing in fact quite close to the sun or moon, and 

 having their colors in reverse order the violet next the sun. 

 The coronae are due to the diffraction and interference of light 



