3 12 
POPULAR SCIENCE REVIEW. 
ing a certain analogy, — but nothing more, — to the undulations of 
sound, which convey to the seat, of sensation the impressions of 
musical harmony. 
The hypothesis that Light is the result of vibrations, esta- 
blished by solar influence on some ethereal medium existing 
throughout planetary space, is now almost universally received. 
It is quite certain that by far the larger number of the pheno- 
mena of Light will admit of explanation, in a satisfactory 
manner, by this hypothesis only. At the same time, it must 
not be disguised that the most zealous advocate of the theory 
of luminous undulations, has failed to show that there is any 
support given to his views by the chemical phenomena of 
Light. It is not intended by this remark to suggest a doubt 
against a doctrine so ably supported ; but it does appear neces- 
sary that there should be found in our philosophical records, at 
least, an attempt to show how chemical changes — photogra- 
phic phenomena — are connected with undulatory motion. 
That Light travels with immense velocity is evident to all. 
The flash of Light from a distant gun reaches the eye long 
before the sound of the explosion is communicated to the ear. 
To measure this velocity it is necessary that we should have the 
means of examining the passage of a ray of Light through vast 
distances, and this has been done by careful observations of 
the eclipses of the satellites of Jupiter. 
Roemer, a Danish astronomer, in 1675, on comparing toge- 
ther observations of eclipses of these satellites during many suc- 
cessive years, observed that the eclipses when Jupiter was 
nearest to the Earth took place too soon, whereas when those 
two planets were at their greatest distance apart, the eclipses 
were always too late ; that is, the eclipses of the moons by the 
planet were, according to the distance from the Earth, either 
sooner or later than by calculation they should have been. 
Speculating on the physical cause of this, Roemer was led to 
beheve in a gradual, instead of an instant propagation of Light, 
and the velocity required to explain these differences between 
observation and calculation was 192,000 miles in every second 
of time. This has since received the fullest confirmation. 
Bradley, by his discovery of the aberration of light (the ap- 
parent displacement of a luminous point by the onward 
motion of any spot on the Earth’s surface), has given us the 
means of proving that the error, if any exists, is an ex- 
ceedingly small one : the most accurate determination giving 
191,515 miles as the velocity of Light in a second. This is a 
speed which it is difficult to realize to the imagination. The 
greatest velocity which has been attained by locomotive engines, 
in some experimental trials on our railways, has been 100 
miles within the hour. If we were to start at this speed from 
