Great Astronomers by Robert Stawell Ball (best ereader for students .txt) 📖

and he measured the amount of the deflection. Again he

tried the same experiment with one of the red rays from the

opposite end of the coloured band. He allowed it to pass through

the same aperture in the screen, and he tested the amount by

which the second prism was capable of producing deflection.

He thus found, as he had expected to find, that the second prism

was more efficacious in bending the violet rays than in bending

the red rays. Thus he confirmed the fact that the various hues

of the rainbow were each bent by a prism to a different extent,

violet being acted upon the most, and red the least.

 

[PLATE: ISAAC NEWTON.]

 

Not only did Newton decompose a white beam into its constituent

colours, but conversely by interposing a second prism with its

angle turned upwards, he reunited the different colours, and thus

reproduced the original beam of white light. In several other

ways also he illustrated his famous proposition, which then seemed

so startling, that white light was the result of a mixture of all

hues of the rainbow. By combining painters’ colours in the right

proportion he did not indeed succeed in producing a mixture which

would ordinarily be called white, but he obtained a grey pigment.

Some of this he put on the floor of his room for comparison with a

piece of white paper. He allowed a beam of bright sunlight to

fall upon the paper and the mixed colours side by side, and a

friend he called in for his opinion pronounced that under these

circumstances the mixed colours looked the whiter of the two.

 

By repeated demonstrations Newton thus established his great

discovery of the composite character of light. He at once

perceived that his researches had an important bearing upon

the principles involved in the construction of a telescope.

Those who employed the telescope for looking at the stars,

had been long aware of the imperfections which prevented all the

various rays from being conducted to the same focus. But this

imperfection had hitherto been erroneously accounted for. It had

been supposed that the reason why success had not been attained

in the construction of a refracting telescope was due to the fact

that the object glass, made as it then was of a single piece,

had not been properly shaped. Mathematicians had abundantly

demonstrated that a single lens, if properly figured, must conduct

all rays of light to the same focus, provided all rays experienced

equal refraction in passing through the glass. Until Newton’s

discovery of the composition of white light, it had been taken for

granted that the several rays in a white beam were equally

refrangible. No doubt if this had been the case, a perfect

telescope could have been produced by properly shaping the object

glass. But when Newton had demonstrated that light was by no

means so simple as had been supposed, it became obvious that

a satisfactory refracting telescope was an impossibility when

only a single object lens was employed, however carefully that

lens might have been wrought. Such an objective might, no doubt,

be made to conduct any one group of rays of a particular shade

to the same focus, but the rays of other colours in the beam of

white light must necessarily travel somewhat astray. In this

way Newton accounted for a great part of the difficulties which

had hitherto beset the attempts to construct a perfect refracting

telescope.

 

We now know how these difficulties can be, to a great extent,

overcome, by employing for the objective a composite lens made of

two pieces of glass possessing different qualities. To these

achromatic object glasses, as they are called, the great

development of astronomical knowledge, since Newton’s time, is

due. But it must be remarked that, although the theoretical

possibility of constructing an achromatic lens was investigated by

Newton, he certainly came to the conclusion that the difficulty

could not be removed by employing a composite objective, with two

different kinds of glass. In this his marvellous sagacity in the

interpretation of nature seems for once to have deserted him.

We can, however, hardly regret that Newton failed to discover

the achromatic objective, when we observe that it was in

consequence of his deeming an achromatic objective to be

impossible that he was led to the invention of the reflecting

telescope. Finding, as he believed, that the defects of the

telescope could not be remedied by any application of the

principle of refraction he was led to look in quite a different

direction for the improvement of the tool on which the advancement

of astronomy depended. The REFRACTION of light depended as he

had found, upon the colour of the light. The laws of REFLECTION

were, however, quite independent of the colour. Whether rays be

red or green, blue or yellow, they are all reflected in precisely

the same manner from a mirror. Accordingly, Newton perceived that

if he could construct a telescope the action of which depended upon

reflection, instead of upon refraction, the difficulty which had

hitherto proved an insuperable obstacle to the improvement of the

instrument would be evaded.

 

[PLATE: SIR ISAAC NEWTON’S LITTLE REFLECTOR.]

 

For this purpose Newton fashioned a concave mirror from a mixture

of copper and tin, a combination which gives a surface with almost

the lustre of silver. When the light of a star fell upon the

surface, an image of the star was produced in the focus of this

mirror, and then this image was examined by a magnifying eye-piece. Such is the principle of the famous reflecting telescope

which bears the name of Newton. The little reflector which he

constructed, represented in the adjoining figure, is still

preserved as one of the treasures of the Royal Society. The

telescope tube had the very modest dimension of one inch in

diameter. It was, however, the precursor of a whole series

of magnificent instruments, each outstripping the other in

magnitude, until at last the culminating point was attained in

1845, by the construction of Lord Rosse’s mammoth reflector

of six feet in aperture.

 

Newton’s discovery of the composition of light led to an

embittered controversy, which caused no little worry to the great

Philosopher. Some of those who attacked him enjoyed considerable

and, it must be admitted, even well-merited repute in the ranks of

science. They alleged, however, that the elongation of the

coloured band which Newton had noticed was due to this, to that,

or to the other—to anything, in fact, rather than to the true

cause which Newton assigned. With characteristic patience and

love of truth, Newton steadily replied to each such attack.

He showed most completely how utterly his adversaries had

misunderstood the subject, and how slight indeed was their

acquaintance with the natural phenomenon in question. In reply to

each point raised, he was ever able to cite fresh experiments and

adduce fresh illustrations, until at last his opponents retired

worsted from the combat.

 

It has been often a matter for surprise that Newton, throughout

his whole career, should have taken so much trouble to expose the

errors of those who attacked his views. He used even to do this

when it plainly appeared that his adversaries did not understand

the subject they were discussing. A philosopher might have said,

“I know I am right, and whether others think I am right or not may

be a matter of concern to them, but it is certainly not a matter

about which I need trouble. If after having been told the truth

they elect to remain in error, so much the worse for them; my time

can be better employed than in seeking to put such people right.”

This, however, was not Newton’s method. He spent much valuable

time in overthrowing objections which were often of a very futile

description. Indeed, he suffered a great deal of annoyance from

the persistency, and in some cases one might almost say from the

rancour, of the attacks which were made upon him. Unfortunately

for himself, he did not possess that capacity for sublime

indifference to what men may say, which is often the happy,

possession of intellects greatly inferior to his.

 

The subject of optics still continuing to engross Newton’s

attention, he followed up his researches into the structure of the

sunbeam by many other valuable investigations in connection with

light. Every one has noticed the beautiful colours manifested in

a soap-bubble. Here was a subject which not unnaturally attracted

the attention of one who had expounded the colours of the spectrum

with such success. He perceived that similar hues were produced

by other thin plates of transparent material besides soap-bubbles,

and his ingenuity was sufficient to devise a method by which the

thicknesses of the different films could be measured. We can

hardly, indeed, say that a like success attended his

interpretation of these phenomena to that which had been so

conspicuous in his explanation of the spectrum. It implies no

disparagement to the sublime genius of Newton to admit that the

doctrines he put forth as to the causes of the colours in the

soap-bubbles can be no longer accepted. We must remember that

Newton was a pioneer in accounting for the physical properties

of light. The facts that he established are indeed

unquestionable, but the explanations which he was led to offer

of some of them are seen to be untenable in the fuller light

of our present knowledge.

 

[PLATE: SIR ISAAC NEWTON’S SUN-DIAL.]

 

Had Newton done nothing beyond making his wonderful discoveries

in light, his fame would have gone down to posterity as one of

the greatest of Nature’s interpreters. But it was reserved for

him to accomplish other discoveries, which have pushed even his

analysis of the sunbeam into the background; it is he who has

expounded the system of the universe by the discovery of the

law of universal gravitation.

 

The age had indeed become ripe for the advent of the genius of

Newton. Kepler had discovered with marvellous penetration the

laws which govern the movements of the planets around the sun, and

in various directions it had been more or less vaguely felt that

the explanation of Kepler’s laws, as well as of many other

phenomena, must be sought for in connection with the attractive

power of matter. But the mathematical analysis which alone could

deal with this subject was wanting; it had to be created by

Newton.

 

At Woolsthorpe, in the year 1666, Newton’s attention appears to

have been concentrated upon the subject of gravitation.

Whatever may be the extent to which we accept the more or less

mythical story as to how the fall of an apple first directed the

attention of the philosopher to the fact that gravitation must

extend through space, it seems, at all events, certain that this

is an excellent illustration of the line of reasoning which he

followed. He argued in this way. The earth attracts the apple;

it would do so, no matter how high might be the tree from which

that apple fell. It would then seem to follow that this power

which resides in the earth by which it can draw all external

bodies towards it, extends far beyond the altitude of the loftiest

tree. Indeed, we seem to find no limit to it. At the greatest

elevation that has ever been attained, the attractive power of the

earth is still exerted, and though we cannot by any actual

experiment reach an altitude more than a few miles above the

earth, yet it is certain that gravitation would extend to

elevations far greater. It is plain, thought Newton, that an

apple let fall from a point a hundred miles above this earth’s

surface, would be drawn down by the attraction, and would

continually gather fresh velocity until it reached the ground.

From a hundred miles it was natural to think of what