Micrographia: Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses with Observations and Inquiries ThereuponHooke, Robert
History
Micrographia: Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses with Observations and Inquiries Thereupon
Hooke, Robert
Magnifying glasses -- Early works to 1800; Microscopy -- Early works to 1800; Natural history -- Pre-Linnean works
Secondly, There will not in this _Hypothesis_ be any other shadow of the
Earth, such as _Kepler_ supposes, and calls the _Penumbra_, which is the
shadow of the refracting _Atmosphere_; for the bending of the Rays being
altogether caus’d by _Inflection_, as I have already shewn, all that part
which is ascribed by _Kepler_, and others after him, to the _Penumbra_, or
dark part, which is without the _umbra terræ_, does clear vanish; for in
this _Hypothesis_ there is no refracting surface of the Air, and
consequently there can be no shadows, such as appear in the ninth Figure of
the 37. _Scheme_, where let ABCD represent the Earth, and EFGH the
_Atmosphere_, which according to _Keplers_ supposition, is like a Sphære of
Water terminated with an exact surface EFGH, let the lines MF, LB, ID, KH,
represent the Rays of the Sun; ’tis manifest, that all the Rayes between
LB, and ID, will be reflected by the surface of the Earth BAD, and
consequently, the conical space BOD would be dark and obscure; but, say the
followers of _Kepler_, the Rays between MF, and LB, and between ID, and KH,
falling on the _Atmosphere_, are refracted, both at their ingress and
egress out of the _Atmosphere_, nearer towards the Axis of the sphærical
shadow CO, and consequently, inlighten a great part of that former dark
Cone, and shorten, and contract, its top to N. And because of this
Reflection of these Rays, say they, there is superinduc’d another shell of
a dark Cone FPH, whose Apex P is yet further distant from the Earth: By
this _Penumbra_, say they, the Moon is Eclipsed, for it alwayes passes
between the lines 12, and 34.
To which I say, That if the Air be such, as I have newly shewn it to be,
and consequently cause such an inflection of the Rays that fall into it,
those dark _Penumbra’s_ FYZQ, HXVT, and ORPS, will all vanish. For if we
suppose the Air indefinitely extended, and to be no where bounded with a
determinate refracting surface, as I have shewn it uncapable of having,
from the nature of it; it will follow, that the Moon will no where be
totally obscured, but when it is below the Apex N, of the dark blunt Cone
of the Earth’s shadow: Now, from the supposition, that the Sun is distant
about seven thousand Diameters, the point N, according to calculation,
being not above twenty five terrestrial Semidiameters from the Center of
the Earth: It follows, that whensoever the Moon eclipsed is totally
darkned, without affording any kind of light, it must be within twenty five
Semidiameters of the Earth, and consequently much lower then any
Astronomers have hitherto put it.
This will seem much more consonant to the rest of the secundary Planets;
for the highest of _Jupiter’s_ Moons is between twenty and thirty _Jovial
Semidiameters_ distant from the Center of _Jupiter_; and the Moons of
_Saturn_ much about the same number of _Saturnial Semidiameters_ from the
Center of that Planet.
Public-domain text, read in full here on John Shaqi.
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