March 1,1873.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
687 
zinc arc consumed. Would the amount of heat generated 
in the battery he the same as before ?. No, it would .be 
less by the precise amount generated in the thin wire 
outside the battery. In fact, by adding the internal 
heat to the external we obtain for the combustion of 100 
grains of zinc a total which never varies. By this ar¬ 
rangement then we are able to burn our zinc at one 
place, and to exhibit the heat and light of its combus¬ 
tion at a distant place. In New \ork, for example, we 
have our grate and fuel; but the heat and light of our 
fire may be made to appear at San Francisco.. We have 
here an illustration of the constant law that in physical 
nature we have incessant substitution but never crea¬ 
tion. 
I now remove the thin wire &nd cittcich to the severed 
ends of the thick one two thin rods of coke. On 
brinffin 0, the rods together we obtain a small star of 
light Now the light to be employed in our lectures is 
a simple exaggeration of this star. Instead of being 
produced by 10 cells it is produced by 50. Placed in a 
suitable camera, provided with a suitable lens, this light 
will give us all the beams necessary for our experiments. 
And here, in passing, let mo refer to the common de¬ 
lusion that the works of nature, the human eye included, 
are theoretically perfect. The degree of perfection, ol 
any organ is determined by what it has to do. Looking 
at the dazzling light from our large battery you see a 
.globe of light, but entirely fail to see the shape of the 
coke-points whence the light issues. Ihe cause may be 
thus illustrated : On the screen before you is now pro¬ 
jected an image of the carbon points, the whole of the 
lens in front of the camera being employed to form the 
image. It is not sharp, but surrounded by a halo which 
nearly obliterates it. This arises from an imperfection 
of the lens, called spherical aberration, due to the fact 
that the circumferential and central rays have not the 
same focus. The human eye labours under a similar 
-defect, and when you looked at the naked light from 
fifty cells the blur of light upon the retina was sufficient 
to destroy the definition of the retinal image of the car¬ 
bons. A long list of indictments might indeed be 
brought against the eye—its opacity, its want of sym¬ 
metry its lack of achromatism, its absolute blindness 
in fact. All these taken together caused an eminent 
Herman philosopher to say that if any optician sent him 
an instrument so full of defects he would send, it back to 
him with the severest censure. But the eye is not to.be 
judged from the standpoint of theory. As a practical in¬ 
strument, and taking the adjustment by which its defects 
are neutralized into account, it must ever remain a marvel 
to the reflecting mind. 
[He now employed the larger battery, using the elec¬ 
tric light as occasion required in any one of three or 
four lanterns in which the carbon points were moved by 
.clockwork. The first image obtained on the screen of 
the carbon points looked like a faint drawing in brown 
pastel on a pure white circular ground. The circular 
ground represented the globular light as seen by the eye. 
A transparent screen was then interposed, which cut. off 
the halo, and the carbon points seemed like two burning 
volcanoes, one upside down and its crater overhanging 
the other, which was conical. Between them there was 
•a space that looked like the burning gas of a blast furnace, 
where the elements would melt with fervent heat.] 
And now we are ready for work. Ihe rectilineal pro¬ 
pagation of light may be beautifullyillustrated by making 
a small hole in a window-shutter, before wffiich stands a 
house, or tree, or animal, and placing within the dark¬ 
ened room a white screen at some distance from the 
orifice. Every straight ray proceeding from the object 
.stamps its colour upon the screen, and the sum of all the 
rays forms an image of the object. But as the rays cross 
.each other at the orifice the image is inverted. An image 
of the carbon points produced by a pinhole in tinfoil will 
be employed to illustrate this point of rectilineal propa¬ 
gation. 
[In each of these images the carbon points wero 
apparent; they gradually overlapped each other as their 
number and proximity was increased, until the audience 
were ready to accept the statement of the lecturer that 
the total circle of light from the lens was merely the 
intricate and complete overlapping of images of the 
carbon.] 
The law that the angle of incidence is equal to the 
angle of reflection is illustrated in this simple way : A 
straight lath is placed as an index perpendicular to a 
small looking-glass capable of rotation. A beam of 
light is received upon the glass and reflected back along 
the line of its incidence. Though the incident and tho 
reflected beams pass in opposite directions, they do not 
jostle or displace each other. The index is now turned, 
the mirror turns along with it, and at each side of the 
index tho incident and the reflected beams are seen 
tracking themselves through the dust of the room. Tho 
mere inspection of the two angles inclosed between the 
index and the two beams shows their equality. Tho. 
same simple apparatus enables us to illustrate a law of 
great practical importance, namely, that when a mirror 
rotates the angular velocity of a beam reflected from it 
is twice that of the reflecting mirror. 
For more than 1000 years no step was taken in optics 
beyond this law of reflection. The men of the middle 
ages, in fact, occupied themselves on the one hand in 
trying to develop the laws of the universe out of their 
own consciousness, while on the other hand they were 
so occupied with the concerns of a future world, that 
they looked with a lofty scorn on all things pertaining 
to this only. As regards the refraction of light, the 
course of real inquiry was resumed in 1100 by an Arabian 
philosopher named Alhazen. Then it was^taken up in 
succession by Roger Bacon, Vitellio and Kepler. One 
of the most important occupations of science is the de¬ 
termination by precise measurements of the quantitative 
relations of phenomena. The value of such measure¬ 
ments depends upon the skill and absolute. conscien¬ 
tiousness of the man who makes them. Vitellio was 
such a man, while Kepler’s habit was to rummage 
through the observations of his predecessors, look, at 
them in all lights, and thus distil from them the prin¬ 
ciple which united them. He had done this with the 
astronomical measurements of Tycho Brahe, and had ex¬ 
tracted from them the celebrated u laws of Kepler. 
But in the case of refraction he was not successful. 
The principle, though a simple one, escaped him. It 
was first discovered by Willcbord Snell, about the year 
1621. 
Less with the view of dwelling upon the phenomenon 
itself than of introducing it to you in a form which wdl 
render intelligible the play of theoretic thought in New¬ 
ton’s mind I will show you the fact of refraction. Ihe 
dust of the air and the turbidity of a liquid may here be 
turned to account. A shallow circular vessel with a 
glass face, half filled ■with water, rendered barely turbid 
by the precipitation of a little mastic, is placed upon its 
edge with its glass face vertical. Through a. slit in tho 
hoop surrounding the vessel a beam of light is admitted. 
It impinges upon the water, enters it, and tracks itself 
through the liquid in a sharp, bright band.. Meanwhile 
the beam passes unseen through the air above the 
water. A puff of tobacco smoke into this space at once 
reveals the track of the incident beam. If the incidence 
be vertical, the beam is unrefracted. If oblique its re¬ 
fraction at the common surface of air and water is ren¬ 
dered clearly visible. It is also seen that reflection ac¬ 
companies refraction, tho beam dividing itself at the 
point of incidence into a refracted and a reflected portion. 
[This experiment was performed with a small drum 
containing liquid. The lecturer remarked that milk 
having a slight bluishness answered the purpose. Lon¬ 
don milk was particularly well suited. There was also 
visible in the smoke above the water a faint reflected 
beam, probably reflected from the surface of the water. 
