to London), by which I could discern Jupiter's 4 Concomitants, and showed them diverse times to two others of my acquaintance. I could also discern the Moon-like phase of Venus, but not very distinctly, nor without some niceness in disposing of the instrument. "From that time I was interrupted until this last autumn when I made the other. And as that was sensibly better than the first (especially for day-objects), so I doubt not, but they will be still brought to much greater perfection by their endeavours, who, as you inform me, are taking care of it at London." Newton's theory of colour After a remark that microscopes seem as capable of improvement as telescopes, he adds: Further on, after some remarks on the subject of compound colours, he says: He concludes his communication with the words: Controversies The publication of these discoveries led to a series of controversies which lasted for several years, in which Newton had to contend with the eminent English physicist Robert Hooke, Anthony Lucas (mathematical professor at the University of Liège), Franciscus Linus (a physician in Liège), and many others. Some of his opponents denied the truth of his experiments, refusing to believe in the existence of the spectrum. Others criticised the experiments, saying that the length of the spectrum was never more than three and a half times the breadth, whereas Newton found it to be five times the breadth. It appears that Newton made the mistake of supposing that all prisms would give a spectrum of the same length; the objections of his opponents led him to measure carefully the lengths of spectra formed by prisms of different angles and different refractive indices, but he was not led thereby to the discovery of the different dispersive powers of different refractive substances. Newton carried on the discussion with the objectors with great courtesy and patience, but the pain which these long discussions gave to his sensitive mind may be estimated from his letter of 18 November 1676 to Oldenburg: "I promised to send you an answer to Mr. Lucas this next Tuesday, but I find I shall scarce finish what I have designed, to get a copy taken of it by that time, and therefore I beg your patience a week longer. I see I have made myself a slave to philosophy, but if I get free of Mr. Lucas's business, I will resolutely