364 THE ILLUMCNATtON OF THE ECLIPSED MOON. 



influence of the corona. The eclipse of Marcli, 1848, which 

 has often been cited as an abnormally bright eclipse, oc- 

 curred (as nearly as I can ascertain) about two and a half 

 years after a sun-spot minimum; and, on the other hand, 

 the exceptionally dark eclipse of October, 1884, occurred 

 about the time of a sun-spot maximum. 



I pass now to a consideration of the matter from another 

 point of view, and propose to inquire whotlier a new light 

 may not be thrown upon it by a careful investigation of the 

 course of the refracted solar rays through the different 

 strata of the earth's atmosphere. 



In the diagram annexed I have indicated the points at 

 which rays from various parts of tlio sun's disc would, 

 after refraction through the lowest stratum of the earth's 

 atmosphere, converge upon the central axis of the shadow. 

 The moon is also shown in its two extreme positions of 

 perigee and apogee. The diagram is drawn to scale so far 

 as regards the relative distances of these several points. 

 The amount of refraction suffered by a ray which grazes 

 the earth, traversing the entire thickness of the terrestrial 

 atmosphere, is taken as one degree six minutes, or double 

 the amount of what is known as " horizontal refraction." 

 A ray from the near limb of the sun, having undergone that 

 amount of refraction, would meet its fellow in the axis of 

 the shadow at the point B, distant from the earth about 

 1G6 370 miles. A ray from the centro of the sun would 

 Strike the axis at D, distant 206,870 miles. And rays from 

 the further li.mh, having undergone what, for distinction 

 may be called cross-refraction, would converge at F, distant 

 273,460 miles. The point B would be the nearest point at 

 which any refracted rays from the sun could reach the 

 centre of the shadow, and the space within the angle 



