1874.] Notices of Books. 99 
uncomfortable. That men in the ranks of science should feel 
thus is, I think, a proof that they have suffered themselves to be 
misled by the popular definition of a great faculty, instead of 
observing its operation in their own minds. Without imagination 
we cannot take a step beyond the bourne of the mere animal 
world, perhaps not even to the edge of this. __ But, in speaking 
thus of imagination, I do not mean a riotous power which deals 
capriciously with facts, but a well ordered and disciplined power, 
whose sole function is to form conceptions which the intellect 
imperatively demands. Imagination thus exercised never really 
severs itself from the world of fact. This is the storehouse from 
which all its pictures are drawn; and the magic of its art consists, 
not in creating things anew, but in so changing the magnitude, 
position, and other relations of sensible things, as to render them 
fit for the requirements of the intellect in the subsensible world.” 
The growth of the rival theories is then traced, and the use of 
imagination by those who favoured one or the other; we were 
unaware that Sir David Brewster did not adopt the undulatory 
theory, and certainly unaware that he permitted sentiment to 
interfere with his acceptation of a physical theory, as the fol- 
lowing passage indicates :—‘‘ In one of my latest conversations 
with Sir David Brewster, he said to me that his chief objection 
to the undulatory theory of light was, that he could not think the 
Creator guilty of so clumsy a contrivance as the filling of space 
with ether in order to produce light. This, I may say, is very 
dangerous ground, and the quarrel of science with Sir David on 
this point, as with many estimable persons on other points, is, 
that they profess to know too much about the mind of the 
Creator.” The elaborate and most due praise which Tyndall, 
and Helmholtz, and many others have ‘bestowed upon Thomas 
Young here reappears. But when Dr. Tyndall tells us that 
Young was but alittle lower in the intellectual scale than Newton, 
and greater than any man of science between Newton’s time and 
his own, we cannot, with all possible love for our country and 
countrymen, hold with him. For he practically makes Newton 
the greatest man of science of all time, and Young second 
only to the greatest. Elsewhere he says ‘‘ we trace the progress 
of astronomy through Hipparchus and Ptolemy ; and after a long 
halt, through Copernicus, Galileo, Tycho Brahe, and Kepler; 
while from the high table-land of thought raised by these men 
Newton shoots upward like a peak, overlooking all others from 
his dominant elevation.” And we would invite comparison 
_ between the great and glorious Leonardo da Vinci and the great 
and glorious Dr. Thomas Young—men who had many remarkable 
points of contact. If no more, at least they should be “bracketed 
together” in the list of illustrious men, but we incline to the 
greater exaltation of Leonardo da Vinci. 
The third lecture treats of the double refracton and polarisation 
of light. We may particularly call attention to Fig. 27, and the 
