The 13th century Venetians glass blowers are known to have produced reading stones made of solid glass that was put into hand-held, single lens-type frames made of horn or wood. These reading stones were similar to hand-held magnifying lenses of today.
The stronger the magnetic field, the larger the magnetosphere. Some 20,000 times stronger than Earth's magnetic field, Jupiter's magnetic field creates a magnetosphere so large it begins to avert the solar wind almost 3 million kilometers before it reaches Jupiter. The magnetosphere extends so far past Jupiter it sweeps the solar wind as far as the orbit of Saturn.
You should be able to see from the table that this gravitational field at the surface of a planet does not just depend on the mass of the planet. For example, if you look at the gravitational field at the surface of Saturn it is the same as that on the surface of Uranus although Saturn is much more massive. The size of this gravitational field is very important if it too large we would be pulled so strongly to the surface that we would be crushed.