888 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[May 10, 1873. 
as Wliewell says, “ was the image of his case. The boat 
was the earth, moving in its orbit, and the wind was the 
light ©f a star.” 
You will immediately understand the meaning of 
Bradley’s discovery. Imagine yourself in a motionless 
railway train with a shower of rain descending vertically 
downward. The moment the train begins to move the 
rain-drops begin to slant, and the quicker the train the 
greater is the obliquity. In a precisely similar manner 
the rays from a star vertically overhead are caused to 
slant by the motion of the earth through space. Know¬ 
ing the speed of the train, and the obliquity of the fall¬ 
ing drops, the velocity of the drops may be calculated; 
and knowing the speed of the earth in her orbit, and the 
obliquity of the rays due to this cause, we can calculate 
just as easily the velocity of light. Bradley did this, and 
the “aberration of light,” as his discovery is called, enabled 
him to assign to it a velocity almost identical with that 
deduced by Roemer from a totally different method of 
observation. 
Up to his demonstration of the composition of white 
light, Newton had been everywhere triumphant — 
triumphant in the heavens, triumphant on the earth, and 
'his subsequent experimental work is, for the most part, of 
immortal value. But infallibility is not the gift of man, 
and, soon after his discovery of the nature of white light, 
Newton proved himself human. He supposed that refrac¬ 
tion and dispersion went hand in hand, and that you could 
not abolish the one without at the same time abolishing 
the other. He maintained this opinion to the end of his 
life, and thus retarded the progress of subsequent discovery. 
Holland at length proved that, by combining together two 
different kinds of glass, you might abolish the colour and 
still leave a residue of refraction ; and he applied this 
residue to the construction of achromatic lenses—lenses 
which yield no colour, which Newton thought an impossi¬ 
bility. 
But Newton committed a graver error than this. 
Science, as I sought to make clear to you in our second 
lecture, is only in part a thing of the senses. The 
roots of phenomena are imbedded in a region beyond 
the reach of the senses, and less than the root of the 
matter will never satisfy the scientific mind. We find, 
accordingly, in this career of optics the greatest minds 
-constantly yearning to pass from the phenomena to their 
causes—to explore them to their hidden roots. They 
thus entered the region of theory, and here Newton, 
though drawn from time to time toward the truth, was 
drawn still more strongly toward the error, and made it 
his substantial choice. His experiments are imperishable, 
but his theory is dead. For a century it stood like a dam 
across the course of discovery ; but, like all barriers that 
rest upon authority and not upon truth, the pressure from 
behind increased, and eventually swept the barrier away. 
This, as you know, was done mainly through the labours 
of Thomas Young and his illustrious French fellow-worker, 
Fresnel. 
In 1808 Malus, looking through Iceland spar at the 
sun reflected from the window of the Luxembourg Palace 
in Paris, discovered the polarization of light by reflection. 
In 1811 Arago discovered the splendid chromatic pheno¬ 
mena which we have had illustrated by plates of gypsum 
In polarized light ; he also discovered the rotation of the 
plane of polarization by quartz-crystals. In 1813 See- 
beck discovered the polarization of light by tourmaline. 
The same year Brewster discovered those magnificent 
bands of colours that surround the axes of biaxial 
crystals. In 1814 Wollaston discovered the ring of Ice¬ 
land spar. 
All these effects, which, without a theoretic clue, 
would leave the mind in a hopeless jungle of pheno¬ 
mena without harmony or relation, were organically 
.connected by the theory of undulation. The theory was 
applied and verified in all directions, Airy being especially 
conspicuous for the severity and conclusiveness of his 
proofs. 
The most remarkable verification fell to the lot of 
the late Sir William Hamilton of Dublin, a profound 
mathematician, who, taking up the theory where Fresnel 
had left it, arrived at the conclusion that at four spe¬ 
cial points at the surface of the ether-wave in double- 
refracting crystals the ray was divided not into two 
parts, but into an infinite number of parts ; forming 
at these points a continuous conical envelope instead of 
two images. No human eye had ever seen this envelope 
when Sir William Hamilton inferred its existence. Turn¬ 
ing to his friend, Dr. Lloyd, he asked him to test experi¬ 
mentally the truth of his theoretic conclusion. Lloyd, 
taking a crystal of arragonite, and following with the 
most scrupulous exactness the indications of theory, 
cutting the crystal where theory said it ought to be cut, 
observing it where theory said it ought to be observed, 
found the luminous envelope which had previously been a 
mere idea in the mind of the mathematician. 
But while I have thus endeavoured in these lectures to 
illustrate before you the power of the undulatory theory 
as a solver of all the difficulties of optics, do I therefore 
wish you to close your eyes against any evidence that 
may arise against it ? By no means ; we leave it to 
others to shrink from the contemplation of truth because 
it may be for the moment disagreeable. You may urge, 
and justly urge, that a hundred years ago another theory 
was held by the most eminent men, and that, as the 
theory then held had to yield, the undulatory theory may 
have to yield also. This is perfectly logical, but let us 
understand the precise value of the argument. In similar 
language a person in the time of Newton, or even in our 
time, might reason thus: “ Hipparchus and Ptolemy, and 
numbers of great men after them, believed that the earth 
was the centre of the solar system. But this deep-set theo¬ 
retic notion had to give way, and the theory of gravitation 
may in its turn have to give way also.” This is just as 
logical as the first argument. Wherein consists the 
strength of the theory of gravitation ? Solely in its com¬ 
petence to account for all the phenomena of the solar sys¬ 
tem. Wherein consists the strength of the theory of 
undulation ? Solely in its competence to disentangle and 
explain phenomena a hundred fold more complex than 
those of the solar system. Be as sceptical, if you like, 
regarding the undulatory theory; if your scepticism be 
philosophical, it will wrap the theory of gravitation in the 
same great doubt. 
Nevertheless this great theory of undulation, like many 
another truth, which in the long run has proved a blessing 
to humanity, had to establish, by hot conflict, its right to 
existence. Great names were arrayed against it. It had 
been enunciated by Hooke, it had been applied by Huy- 
ghens, it had been defended by Euler. But they made 
no impression. And, indeed, the theory in their hands 
was more an analogy than a demonstration. It first took 
the form of a demonstrated verity in the hands of Thomas 
Young. He brought the waves of light to bear upon each 
other, causing them to support each other, and to extinguish 
each other at will. From their mutual actions he deter¬ 
mined their lengths, and applied his determinations in all 
directions. He showed that the standing difficulty of 
polarization might be embraced by the theory. 
After him came Fresnel, whose transcendent mathe¬ 
matical abilities enabled him to give the theory a gene¬ 
rality unattained by Young. He grasped the theory in 
its entirety; followed the ether into its eddies and estuaries 
in the hearts of crystals of the most complicated structure, 
and into bodies subjected to strains and pressures. He 
showed that the facts discovered by Malus, Arago, Brew¬ 
ster, and Bort were so many ganglia, so to speak, of his 
theoretic organism, deriving from it sustenance and 
explanation. With a mind too strong for the body with 
which it was associated, that body became a wreck long 
before it had become old, and Fresnel died, leaving behind 
him a name immortal in the annals of science, 
(To be continued.) 
