ECLIPSES. 



the horizontal refraction. But the mean value of the horizontal 

 refraction being 33', the mean value of the angle m e m' will 

 be 66'. But since the greatest value of m e o is 45' 42", it follows 

 that the refracted ray e m' will fall upon the section of the shadow 

 at a point beyond its Centre ; and since the same will take place 

 at all points round the shadow, it follows that the entire section 

 will be more or less illuminated by the light thus refracted : the 

 intensity of such illumination increases from the centre towards 

 the borders. 



46. When the moon's limb first enters the shadow at wi, the 

 contrast and glare of the part of the disc still enlightened by the 

 direct rays of the sun render the eye insensible to the more feeble 

 illumination produced upon the eclipsed part of the disc by the 

 refracted rays. As, however, the eclipse proceeds, and the 

 magnitude of the part of the disc directly enlightened decreases, 

 the eye, partly relieved from the excessive glare, begins to 

 perceive very faintly the eclipsed limb, which is nevertheless 

 visible from the beginning in a telescope, in which it appears 

 with a dark grey hue. When the entire disc has passed into 

 the shadow, it becomes distinctly visible, showing a gradation of 

 tints from a bluish or greenish on the outside to a gradually 

 increasing red, which, further in, changes to a colour resembling 

 that of incandescent iron when at a dull red heat. As the lunar 

 disc approaches the centre of the shadow, this red line is spread 

 all over it. Its illumination in this position is sometimes so 

 strong as to throw a sensible shadow, and to render distinctly 

 visible in the telescope the lineaments of light and shadow 

 upon its surface. 



These effects are altogether similar to the succession of tints 

 developed in our atmosphere at sunset, and arise, in fact, from 

 the same cause, operating, however, with a two-fold intensity. 

 The solar rays traversing twice the thickness of air, the blue and 

 green lights are more effectually absorbed, and a still more 

 intense red is imparted to the tints transmitted. Without 

 pursuing these consequences further here, the student will find 

 no difficulty in tracing them in the effects of sunset and of 

 sunrise, and of evening and morning twilight. 



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