160 MALUS. 
INVENTION OF THE REPEATING GONIOMETER. 
Physical theories and experimental methods have a 
mutual reaction on each other. The former cannot be 
which “ would probably long remain to mortify the vanity of an 
ambitious philosophy, completely unresolved by any theory.” 
Again, in a review of Malus’s paper (in 1811), he considers it “ con- 
clusive with respect to the insufficiency of the undulatory theory in its 
present state, for explaining all the phenomena of light.’”’ And again, 
in a letter to Sir David Brewster, five years later, he expresses himself 
thus: “ With respect to my fundamental hypotheses respecting the 
nature of light (7. e. the wave theory), I become less and less fond of 
dwelling on them, as I learn more and more facts like those which 
M. Malus discovers; because, though they may not be incompatible with 
those facts, they certainly give no assistance in explaining them.” 1 Even 
Malus himself was at first of opinion that the phenomena of polariza- 
tion were equally irreconcilable with both the undulatory and mole- 
cular theories; an opinion which he distinctly expressed in a letter 2 
to Young. 
Somewhat later, however, we find Young beginning to entertain a 
more satisfactory view of the case, as appears by the following pas- 
sage from a letter addressed by him to Arago in 1817: ‘1 have been 
reflecting upon the possibility of giving an imperfect explanation of the 
affection of light which constitutes polarization, without departing 
from the genuine doctrine of undulations. It is a principle of this 
theory that all undulations are simply propagated through homoge- 
neous mediums in concentric spherical surfaces, like the undulations 
of sound, consisting simply of the direct and retrograde motions of 
their particles in the direction of the radius, with their concomitant 
condensations and rarefactions. And yet it is possible to explain in 
this theory a transverse vibration, propagated also in the direction of 
the radius, and with equal velocity; the motions of the particles bear- 
ing a certain constant direction with respect to that radius; and this 
is polarization.”’ 8 
Now that the idea of transverse vibrations has become familiarized, 
it seems to present little difficulty; yet it was at first opposed to the 
prepossessions even of the most zealous undulationists. Fresnel long 
1 Dean Peacock’s Life of Young, p. 379. 
2 Works, vol. i. p. 248, note. 
8 Life, p. 890. 
