168 NOTE ON THE TOTAL ECLIPSE OF THE MOON. 
visible ; while, on the other hand, if the atmosphere be highly 
saturated, the blue rays are more effectually absorbed, and the red 
rays transmitted to the moon, thus rendering it visible. The parti 
of the atmosphere through which the rays pass are sometimes 
saturated to different extents ; hence some parts of the disc are seen 
better illuminated than others.” 
To take the last part of this quotation first, it may be well 
to explain that the main differences in the illumination of the 
different parts of the eclipsed moon are undoubtedly due to the 
position of the moon in the earth’s shadow. The portions of the 
moon’s disc nearest the edge of the shadow are the brightest, 
those most deeply immersed are the darkest. Hence in the 
first part of the total phase the preceding or eastern portion of 
the disc is the darker, the following or western portion is the 
brighter ; while in the latter part of the total phase the reverse 
obtains. In the middle of the total phase, if the moon pass 
above the centre of the shadow, the upper portion will be the 
brighter ; if it pass below the centre, the lower portion will be 
the brighter. If the eclipse be nearly central, then in the 
middle of totality the central parts of the moon’s disc will be 
somewhat darker than the surrounding parts. These are the 
appearances which should on theory be observed if the source 
of illumination is tolerably uniform around the earth’s circum- 
ference, and according to my own observations they are the 
appearances which are actually observed with little or no 
variation. The intrinsic differences in the brightness of the 
different parts of the moon’s surface — differences familiar to 
everybody — must of course be also taken into account. I 
doubt if there is sufficient evidence of inequalities of illumina- 
tion of the different parts of the moon’s disc beyond what may 
be referred to one or other of these explanations. 
Passing to the more essential part of the theory stated in 
