8 
when I was at Cambridge readiag an old book at the Union 
there, in which it was stated that the old Indian magicians, 
when any one came to ask for prophesies about some journey 
or undertaking, would take out the enquirer to a moonlit 
plain, and tell him to look steadily at his own shadow, then 
after awhile at the sky, where a luminous image of monstrous 
size would be seen (if at least the man’s retina was of the 
sensitive sort). ' What do you see ? ’ — ‘ A great bright being 
in the sky ? ’ — ' It is your genie ! ’ — ' But has it a head ? ’ 
If the image was perfect in this respect the journey or 
undertaking would proceed ; if otherwise, not. Of course, 
this would really depend on the part of his shadow the 
victim had looked at, for, as you know, a complimentary 
image is apt to be incomplete in parts away from the centre 
of vision.” 
From this it will be seen that there is nothing new in the 
observations. But although not new it is somewhat strange 
that the effect is not commonly seen. A pretty and simple 
way to exhibit the effect is to place a piece of, say, bright 
green paper, an inch square, on a piece of white paper — this, 
if steadily gazed at for some seconds, at 12 inches from the 
eye, will produce an image the same size in complimentary 
colour when the eyes are turned aside. If now another 
piece of white paper 18 inches distant be looked at, an en- 
larged image is at once seen, and the reverse effect is instantly 
visible if the paper is brought within 6 inches of the eyes. 
The experiment, although proving what must happen, is not 
the less curious and interesting. 
“ On the meaning of Addition and Subtraction in Logic,” 
by Joseph John Mukphy, Esq. Communicated by the 
Eev. Kobert Harley, F.E.S., Corresponding Member of the 
Society. 
In Boole’s Logical Algebra, the literal symbols x, y, 0 , 
