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For a telescope to make a telescope, supposes it to select copper and zinc in due proportions and fuse them into brass; to fashion that brass into inter-entering tubes; to collect and combine the requisite materials for the different kinds of glass needed; to melt them, grind, fashion, and polish them; adjust their densities and focal distances, etc., etc.
But after I had begun to consider the inequalities of the lunar motions, and had entered upon some other things relating to the laws and measures of gravity, and other forces; and the figures that would be described by bodies attracted according to given laws; and the motion of several bodies moving among themselves; the motion of bodies in resisting mediums; the forces, densities, and motions of mediums; the orbits of the comets, and such like; I put off that publication till I had made a search into those matters, and could put out the whole together.
If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form.
The following explanation is suggested to account for the origin of the rays: "The earth and her satellite may differ not so much as regards volcanic action as in the densities of their atmospheres.
While I was looking at it, the form of the lake altered sufficiently to have dispelled the illusion, if I had been labouring under it. In a few minutes it passed away altogether, but only to reappear elsewhere. Another curious effect, and rather absurd mistake, resulted from the different densities in the super-heated atmosphere which caused this mirage.
There must come a point where these two attractions would neutralize each other: the projectile would possess weight no longer. If the moon's and the earth's densities had been equal, this point would have been at an equal distance between the two orbs.
In this form it states that a ray of light in transition between two media of different densities is refracted at their boundary surface so that the ratio of the angle which is formed by the ray in either medium with a line at right angles to the boundary surface is such that the quotient of the sines of both angles is for these media a constant factor. In symbols sin α / sin β = c.
'When the first of these was compared with the other three, the effect was most striking. It was drawn towards the axis when the magnet was excited, the tube containing the rarer gas being apparently driven away, and the greater the difference between the densities of the two gases, the greater was the energy of this action. And now observe his mode of reaching a material magnetic zero.
If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form.
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