Clxxii PROCEEDINGS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
Every Science, as we shall see, has associated with it one or more 
of the Arts, which are dependent on it for their guidance, although 
in many cases the Art may be older than the Science. It would be 
impossible even to allude to all of these, but I shall merely mention 
one Chemical Art, whose existence is coincident with the reign, 
namely, Photography. It was in 1839 that Daguerre made his first 
sun-picture, and since that date a constant succession of improve¬ 
ments have taken place and fresh discoveries have been made, 
culminating in the so-called colour-photography and the radiograms 
by means of the Rontgen Rays. 
II.—PHYSICS. 
Under this heading—which includes, perhaps, the most extensive 
group of the sciences—it will be impossible to do more than indicate 
some of the more striking advances. Here, again, the foundations 
had been firmly established, chiefly in the direction of mathematical 
principles, so that progress has been principally in the direction of 
more exact experimentation,—rendered possible by immensely more 
accurate instruments,—and also in daring generalisations. We shall 
glance in turn at the three more important departments of Physics. 
(a) Light.— In 1837 the Interference of light and the Polarisa¬ 
tion of light by reflection had already been demonstrated by Young, 
Fresnel, and Malas, but it remained for subsequent workers to build 
up on these principles the new and marvellous science of Spectro¬ 
scopy. Amongst those who have been foremost in this work are 
Fox-Talbot, Bunsen, Kirchhoff, and Crookes. The result of their 
labours has been an immense addition to our knowledge of the 
Universe, both as regards its constitution and its history, and that 
not only for the substances which go to make up our earth, but for 
the heavenly bodies as well. Indeed, the two factors which, during 
the Victorian Era, have done most to enlarge our conceptions of the 
wonders of creation are the perfecting of the spectroscope and the 
acceptance of the law of development. 
In 1856 Helmholtz, the German Physicist, demonstrated what 
had previously been hinted at by Young, namely, the way in which 
the sensation of colour is produced in the retina of the eye, by the 
combination of different wave-lengths of light, producing the sen¬ 
sations of red, green, and violet respectively. It is on this principle 
that the new colour-photography is founded. 
(b) Heat.— The most important discovery in this department of 
Physics during the Queen’s reign was that made by Dr. Joule of 
Manchester in 1849, formulated in what is known as the Dynamical 
Theory of Heat. This discovery, important as it is in itself, has led 
to a still more important generalisation, namely, the Conservation of 
Energy, which is one of the most far-reaching principles which have 
been laid down in modern times. The dynamical theory of heat, 
which has been still further worked out by Helmholtz, Kelvin, 
Crookes, and other physicists, has given us a new and marvellous 
conception of matter. These investigators have demonstrated that a 
gas, such as hydrogen, consists of molecules so small that it would 
