ADDRESS. lvii 
So much in illustration of the present phase of scientific thought in refer- 
ence to the Newtonian axiom. 
The progress of knowledge of the form of all-pervading force, which 
we call, from its most notable effect on one of the senses, ‘light,’ has not 
been less remarkable than that of gravitation. 
Galileo’s discovery of Jupiter's satellites supplied Rémer with the pheno- 
mena whence he was able to measure, in 1676, the velocity of light. Des- 
cartes, in his theory of the Rainbow, referred the different colours to the 
different amount of refraction, and made a near approximation to Newton’s 
capital discovery of the different colours entering into the composition of the 
luminous ray, and of their different refrangibility. Hook and Huyghens, 
about the same period had entered upon explanations of the phenomena of 
light conceived as due to the undulations of an ether, propagated from the 
luminous point spherically, like those of sound. Newton, whilst admitting 
that such undulations or vibrations of an ether would explain certain phe- 
nomena, adopted the hypothesis of emission as most convenient for the 
mathematical propositions relative to light. The discoveries of achromatism, 
of the laws of double refraction, of polarization circular and elliptical, and 
of depolarization, rapidly followed, realizing more than Bacon conceived 
might flow from the labours of the ‘perspective house,’ and, with later ad- 
vances in optics, have made renowned the names of Dollond, Young, Malus, 
Fresnel, Arago, Biot, Brewster, Stokes, and Jamin. 
Some of the natural sciences, as we now comprehend them, had not ger- 
minated in Bacon’s time. Chemistry was then Alchemy: Geology and Pa- 
leontology were undreamt of: but Magnetism and Electricity had begun to 
be observed, and their phenomena compared and defined by a contem- 
‘porary of Bacon, in a way that claims to be regarded as the first step toward a 
scientific knowledge of those powers. It is true that, before Gilbert*, the 
magnet was known to attract iron, and the great practical application of 
magnetized iron—the mariner’s compass—had been invented, and for many 
years before Bacon's time had guided the barks of navigators through track- 
less seas. S 
Gilbert, to whom the name ‘electricity’ is due, observed that that force 
attracted light bodies, whereas the magnetic force attracted iron only. 
About a century later the phenomena of repulsion as well as of 
attraction of light bodies by electric substances were noticed; and Dufay, 
in 1733, enunciated the principle that “electric bodies attract all those that 
‘are not so, and repel them as soon as they are become electric by the 
vicinity of the electric body.” 
The conduction of electric force, and the different behaviour of bodies in 
contact with the electric, leading to their division, by Desaguliers, into con- 
ductors and non-conductors, next followed. The two kinds of electricity, at 
first by Dufay, their definer, called ¢ vitreous ’ and ‘resinous,’ afterwards, by 
* De Magnete (1600). 
