164 TRANSIT OF MERCURY, 
observers failed to see it, though trained and furnished with good 
telescopes. This rather shook my faith in the irradiation theory. 
At the recent transit of Mercury, as I have just read to you, those 
convinced that one must not look to irradiation for an explanation. 
Tn the first place the amount of this depends upon the brilliance 
of the sun, and when the sun is looked at in the telescope, by 
reflection or coloured glasses all this brilliance is cut off, and there 
is wanting the co ondition for irradiation ; for, be it remembered, 
it is not the actual brilliance of a body, but so much of it as is 
or vibratory motion in the body looked at. We are all fam 
with the persistence of vision which carries to our minds the 
image of something when there is nothing but the ne through 
which something passed, of which passage our persistent vision 
gives us a misleading impression. If a black body be placed in 
front of a white one and made to vibrate rapidly, to our eyes it at 
— enlarges, the space round it becoming a grey halo, aureola 
_ or fringe. Now, if we have the limb of the sun bounded by the 
relatively black oe ia and a black ee a dise on the sun 
tinctly between the two limbs a triangular piece wi 
Reed off the points of the cusps, and seems to make the 
cling to the limb, and in some cases make it like the letter Be 
Now this vibratory motion in the atmosphere is about + a 
pager difficulty of the nica Jt is seldom absent; ane 
ot to be wondered at when we consider that any peor 
‘ietiecnts of unequal sienpababare or dry and moist prewere equal 
at once cause such vibratory motion, ane is simply u” 
refraction of the rays of light passing throug ents, Ue black 
drops, &c., are such common phen at planctary ts menot 
What is wanted is, that we should bo familiae withthe phena@en 
# A steel wire held firmly in a vice carried at the free end a beget ret 
part of which was made black to represent the sky, while we ve 
taken for a part of the sun’s limb was a round black spo Janet and sky 
edge of the white. When the wire is coo to vibrate, the P 
seem to join and form a perfect black 
