The final version of de Dominis’s optical treatise De radiis visus et lucis in vitris perspectivis et iride tractatus, completed by the end of 1609 and the beginning of 1610, was marked by his contribution to the controversy on the telescope independently of Galileo and prior to Kepler and Clavius, devoted entirely to the theory of instrument. Close study of the dedication written by Giovanni Bartoli, the editor, and the Jesuit archival sources shows that de Dominis’s early writings on optics date from the period when the young Jesuit lectured mathematics at Collegium Patavinum in Padua (1588–1590) and was professor of mathematics and philosophy at Collegium Brixiense in Brescia (1592–1595). Bartoli’s dual characterization of these early writings, referred to as notes (commentaria) or treatise (tractatus), leaves us with a dilemma concerning their nature. Alluding to his contribution to the theory of the telescope, de Dominis concluded the chapter on the new scientific instrument with the statement »I am the first to break the ice«. So right he was because the imprimatur of his treatise De radiis was dated January 27, 1610, or 32 days before Galileo’s astronomical report Sidereus nuncius. While submitting his manuscript before the censorship, de Dominis was not acquainted with Galileo’s writing and the latter’s parallel efforts directed towards the application of telescope in astronomical observations and not the theory and the construction of instrument. Although de Dominis’s De radiis was published in October 1611 at the earliest, more than a year and a half after Galileo’s Sidereus nuncius, the comparative textual analysis of the first pages of Galileo’s pioneering work and de Dominis’s two indirect objections concerning Galileo leads to the conclusion that de Dominis made no changes in his manuscript after the publication of Galileo’s Sidereus nuncius. That is why de Dominis’s methods and conclusions in De radiis should be viewed in accordance with status quaestionis of the end of 1609. Even if Galileo’s public demonstration of the telescope on August 25, 1609, did influence the final version and publishing of de Dominis’s treatise, it retained the structure of de Dominis’s approach to optical problems from the time of his professorship (1588-1595): the foundations of geometrical and physical optics, an abridged theory of optical lenses, explanation of the rainbow. In January 1610 at the latest, the theory of lenses was accompanied by a short yet valuable contribution to the theory of the telescope, the first of the kind written after August 25, 1609. In the »first« part of the treatise de Dominis tried to list all the physical and mathematical suppositions and propositions that he would refer to during the optical research. Some of his statements were true, some incorrect or obscure with exactly described effects. Certain statements were added afterwards in places where they could contribute to the proof. He followed a similar pattern with the basic notions as well. For example, he defined the notion of the angle of view but failed to do the same with the focal length. After an obscure explanation of the origin of vision, in the »second« part de Dominis expounded the optical lenses despite his scanty knowledge of the basic optical notions; for example, he acquired his understanding of the notion of focal length and focus in the course of the experiments. Moreover, his conclusions on the lenses are entirely based on the experiments. Two chapters on the telescope were neither conceived nor realized as a text which would argue for or against Galileo, thus crediting de Dominis’s treatise with a privileged position, undetected in the historical or philosophical study of Galileo’s epoch. Although de Dominis’s text on the telescope is theoretical and promotive, the distinguished nobleman of Rab harboured doubts about the possible improvements of the instrument regarding its main effect – magnification of an object. Thus he contradicted one of the basic intentions of his treatise – »the marvellous effects« (effectus mirabiles) of the lenses. In his explanation of the rainbow, comprised in the »third« part of the treatise, de Dominis decided on three approaches: a historical survey of the interpretation of the rainbow from Aristotle to the natural philosophers of the Renaissance, commentary on the third book of Aristotle’s Meteorologica, and the original scientific dispute (nova et propria disputatio), based on his own experiments and explanations. Following Aristotle’s starting-point expressed in »Reflection is the cause of the rainbow«, he offered his own interpretations of the origin of the primary and secondary bow. In them the original contribution was accompanied by occasional lack of clarity and certain weaknesses in the illustration of the experiment. De Dominis’s interpretation of the bow proved incomplete where least expected: he was inconsistent in the application of the proposition on the refraction of light on the boundary between two transparent media, although he had clearly formulated it at the beginning of the treatise. In addition, while describing the experiments with the small glass ball filled with water, he failed to distinguish the path of a single light beam and the representation of the ray bundle. De Dominis’s ever-lasting dilemma of whether to follow in the experiment a single ray of light or their bundle made his theoretical explanation and the illustration of the key experiment incomprehensible. On the pages of his optical treatise the natural philosopher and scientist, de Dominis, combined theory and experiment more frequently than many of his specialists were ready to admit. True, commitment to the experimental result did at times lead towards erroneous theoretical conclusions, just as adherence to a certain theoretical view interfered with the accurate interpretation of the experiment. De Dominis’s optical research was characterized by constant tension between theory and experiment, the result being unpredictible.