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But Galileo's great scientific contribution is not limited to the worlds revealed through the telescope which was ever-developing by he himself. He was a messenger of the stars, who never ceased to occupy himself with our earthly world as well. So, he showed particular interest in the phenomena of mechanics. He studied not only the parabolic course of projectiles, but also the motion of a body on a slope. Also observing a lamp in the cathedral of Pisa, he proclaimed the principle that the oscillations of a pendulum are isochronous, which proved extremely useful for measuring time.  

After all, reversing the Aristotelian beliefs and contrary to common sense, Galileo proved that all bodies, whether consisting of iron or down, fall - if, of course, the air resistance is ignored – at the same speed. Galileo combined the perspicacity of observation with experimental investigation and also the deep knowledge of mathematics. Hence, he laid the foundation, perhaps without realising it himself, for modern experimental physics.  

Given the breadth of his work and the fact that the telescope was an important application of the properties of light, initially it seems paradoxical that light itself preoccupied Galileo’s mind very little. Being more a practical than a theoretical mind, he was more interested in the proper functioning of the telescope and less on how it works. Through his great intuition, however, he was led to the perception that light, just like heat, can be more easily understood as particulate and mechanical entities.  

Thus, he comes to the conclusion that when a substance is raised to "the real indivisible atoms", then "light is produced". Therefore light can not be excluded from being some corpus like the others; it is just smaller, or perhaps the smallest of all. But its true nature escapes us. Shortly before his death, Galileo even declares that as regards the substance of light, he is still in the dark. Three centuries later and after having decisively contributed to the understanding of light, Albert Einstein will declare the same thing.

Although Galileo referred to light and its properties very little, the turn which he wants light to be studied as an existing corpus with some real properties and not as a divine or spiritual subsistence, is clearly shown in his work. Also Galileo’s personage is imprinted with the continuing struggle of the scientific spirit against the religious dogma and oppression, especially of the Roman Catholic Church. (...)