First of all comes the great cost. Secondly, since
the lenses are held in position merely round their rims, they will bend
by their weight in the centres if they are made much larger. On the
other hand, attempts to obviate this, by making the lenses thicker,
would cause a decrease in the amount of light let through.

But perhaps the greatest stumbling-block to the construction of larger
telescopes is the fact that the unsteadiness of the air will be
increasingly magnified. And further, the larger the tubes become, the
more difficult will it be to keep the air within them at one constant
temperature throughout their lengths.

It would, indeed, seem as if telescopes are not destined greatly to
increase in size, but that the means of observation will break out in
some new direction, as it has already done in the case of photography
and the spectroscope. The direct use of the eye is gradually giving
place to indirect methods. We are, in fact, now _feeling_ rather than
seeing our way about the universe. Up to the present, for instance, we
have not the slightest proof that life exists elsewhere than upon our
earth. But who shall say that the twentieth century has not that in
store for us, by which the presence of life in other orbs may be
perceived through some form of vibration transmitted across illimitable
space? There is no use speaking of the impossible or the inconceivable.
After the extraordinary revelations of the spectroscope--nay, after the
astounding discovery of Roentgen--the word impossible should be cast
aside, and inconceivability cease to be regarded as any criterion.

[8] The principle upon which the telescope is based appears to have been
known _theoretically_ for a long time previous to this. The monk Roger
Bacon, who lived in the thirteenth century, describes it very clearly;
and several writers of the sixteenth century have also dealt with the
idea. Even Lippershey's claims to a practical solution of the question
were hotly contested at the time by two of his own countrymen, _i.e._ a
certain Jacob Metius, and another spectacle-maker of Middleburgh, named
Jansen.

CHAPTER XI

SPECTRUM ANALYSIS

If white light (that of the sun, for instance) be passed through a glass
prism, namely, a piece of glass of triangular shape, it will issue from
it in rainbow-tinted colours. It is a common experience with any of us
to notice this when the sunlight shines through cut-glass, as in the
pendant of a c