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THE PHARMACEUTICAL JOURNAL AND TRANSACTION'S. 
[March 15, lST?. 
upon the filter in small quantities, and after all the crys¬ 
tals had dissolved, the filter was filled with chloroform 
twice to extract any cantharidin which might have been 
absorbed by the paper. The chloroformic filtrate was 
collected in a tared capsule, evaporated spontaneously, 
and the residue weighed. 
In this way 6-097 grains of pure cantharidin of merely 
a pale yellow colour were obtained, which is equal to 
1*016 per cent. If two-fifths per cent, of cantharidin 
is regarded as the yield of fair medicinal cantharides, it 
will he seen that the Mylabris is two and one-half times 
stronger ; it is fully twice the strength of the best com¬ 
mercial cantharides, and it yields 1*78 times more can¬ 
tharidin than the best yield recorded. 
PROFESSOR TYNDALL ON LIGHT. * 
(Continued from p. 688.) 
We might vary and extend our experiments on light 
indefinitely, and they certainly would prove us to possess 
a wonderful mastery over the phenomena. But the ves¬ 
ture of the agent only would thus he seen, not the agent 
itself. The human mind, however, is so constituted and 
so educated as regards natural things, that it can never 
rest satisfied with this outward view of them. 
Let us, then, inquire what this thing is that we have 
been generating, reflecting, refracting, and analysing. 
In doing this we shall learn that the life of the experi¬ 
mental philosopher is two-fold. He lives, in his vocation, 
a life of the senses, using his hands, eyes, and ears, in his 
experiments; hut such a question as that now before us 
carries him beyond the margin of the senses. He cannot 
consider, much less answer, the question, “ What is 
light?” without transporting himself to a world which 
underlies the sensible one, and out of Which, in accordance 
with rigid law, all optical phenomena spring. To rea¬ 
lize this subsensible world, if I may use the term, the 
mind must possess a certain pictorial power. It has to 
visualize the invisible. It must he able to form definite 
images of the things which that subsensible world con¬ 
tains ; and to say that, if such or such a state of things 
exist in that world, then the phenomena which appear 
in. ours must, of. necessity, grow out of this state of 
things. If the picture he correct, the phenomena are 
accounted for; a physical theory has been enunciated 
which uritesand explains them ail. 
This conception of physical theory implies, as you per¬ 
ceive, the exercise of the imagination. If we are to get 
anything done in physical science, we must invoke this 
faculty of imagination. Without it we cannot take a 
step beyond the bourne of the mere animal world, per¬ 
haps not even t o the edge of this. But, in speaking of 
imagination, I do not mean that riotous power which 
those good and timid people appear to be alone acquainted 
with, but a well-ordered and disciplined power whose 
sole function is to form the conceptions which the in¬ 
tellect imperatively demands. Imagination thus exer¬ 
cised 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 creatine 
things anew, but in so changing the magnitude, position! 
and other relation of sensible things, as to render them 
fit for the requirements of the intellect in the subsensible 
world. 
^One of my objects in these lectures is to show you by 
what processes, or in what way, the scientific m'ind at¬ 
tains the deepest knowledge it is capable of attaining, 
which is a knowledge of the invisible sources of pheno¬ 
mena. I will take, as an illustration of this subject, the 
case of Newton. Before ho began to deal with light, he 
wa.s intimately acquainted with the laws of elastic col¬ 
lision, which all of you have seen more or less perfectly 
* Abstract of a 'series of lectures delivered in the Cooper 
Institute, New York, and reported in the New York 
Tribune. 
illustrated on a billiard-table. As regards the collision 
of sensible masses, Newton knew the angle of incidence 
to be equal to the angle of reflection, and he also knew 
that experiment had established the same law with regard 
to light. He thus found in his previous knowledge the- 
material for theoretic images. He had only to change- 
the magnitude of conceptions already in his mind to- 
arrive at the Emission Theory of Light. He supposed 
light to consist of elastic particles of inconceivable 
minuteness shot out with inconceivable rapidity by lu-- 
minous bodies. Such particles impinging upon smooth. 
surfaces were reflected in accordance with the ordinary 
law of elastic collision. The fact of optical reflection 
certainly occurred as if light consisted of elastic particles, 
and this was Newton’s sole justification for introducing*, 
them. 
But this is not all. In another important particular,, 
also, Newton’s conceptions regarding the nature of light 
were influenced by his previous knowledge. He had 
been working at the phenomena of gravitation, and had 
made his mind at home amid the operations of this uni¬ 
versal power. Perhaps that mighty mind at this time 
was too freshly and too deeply imbued with these 
notions to enable it to form an unfettered judgment re¬ 
garding the nature of light. Be that as it may, Newton 
saw in refraction the action of an attractive force exerted 
on the particles of light. He carried his conception 
out with the most severe consistency. Dropping ver¬ 
tically downward toward the earth’s surface, the motion, 
of a body is accelerated as it approaches the earth. 
Dropping in the same manner downward on a horizontal 
surface, say of glass or water, the velocity of the light 
particles, when they came close to the surface, was, ac¬ 
cording to Newton, also accelerated. Approaching 
such a surface obliquely, he supposed the particles, when * 
close to it, to be drawn down upon it, exactly as a pro¬ 
jectile is drawn by gravity to the surface of the earth. 
This deflection by an attractive force exerted upon the- 
particles of light, was, according to Newton, refraction. 
Finally, it was supposed that differences of colour might 
be due to differences in the sizes of the particles; a 
big particle, by its impact against the retina, producing 
one colour, and a small particle a different one. This, 
was the physical theory of light enunciated and defended 
by Newton; and you will observe that it simply consists 
in the transference of conceptions born in the world of 
the senses to a subsensible world. 
But though the region of physical theory lies thus be¬ 
hind the world of senses, the verifications of theory 
occur in that world. Laying the theoretic conception at 
the root of matters, we determine by rigid deduction 
what are the phenomena which must of necessity grow 
out of this root. If the phenomena thus deduced agree 
w r ith those of the actual world, it is a presumption in 
favour of the theory. If as new classes of phenomena 
arise they also are found to harmonize with theoretic 
deduction, the presumption becomes still stronger. If, 
finally, the theory confers prophetic vision upon the 
investigator, enabling him to predict the existence of 
phenomena which have never yet been -seen, and if thoso 
predictions be found on trial to be rigidly correct, the 
persuasion of the truth of the theory becomes overpower¬ 
ing. Thus working backward from a limited number of 
phenomena, genius, by its own expansive force, reaches. 
a conception which covers all the phenomena. There is 
no more wonderful performance of the intellect than this. 
And we can render no account of it. Like the scriptural 
gift of the Spirit, no man can tell whence it cometh. 
The passage from fact to principle is sometimes slow, 
sometimes rapid, and at all times a source of intellectual 
joy. When rapid, the pleasure is concentrated, and 
becomes a kind of ecstasy or intoxication. To any one 
who has experienced this pleasure, even in a moderate 
degree, the action of Archimedes when he quitted the- 
bath, and ran naked, crying “Eureka!” through the-, 
streets of Syracuse, becomes perfectly intelligible. 
