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HISTORICAL VIEW OF THE

In the “ Optics,” we have the complete theory of therainbow, in which the author applies his discovery ofthe unequal but constant indices of refraction belongingto the different rays to complete what was wanting inDes Cartes ’ explanation of the phenomenon. To thesame work also is appended that rich collection of philo-sophic hints and suggestions, under the title of queries,in which so many singular anticipations of after dis-coveries are found : nor is it any disparagement to theauthor, if in some of these he was less fortunate thanin others. His observations on the double refractionof the Iceland crystal, in which he was misled by someinaccurate measures into conclusions at variance withthe law so accurately established by Huyghens , must beclassed among these. On the other hand, he distinctlythrew out the suggestion, that under certain circum-stances a ray of light has different properties on itsdifferent sides, which he referred to somethinganalogous to polarity in its particles: an idea which,under a somewhat different aspect, has been recognisedthroughout an immense class of phenomena elicited bymodern research.

We have here followed the connection of subject,rather than the order of time, in order to give, in onepoint of view, a general idea of Newton’s optical dis-coveries : and it must on all hands be admitted, thatscarcely a parallel case can be produced in the annalsof science, where a single individual has at once ori-ginated the idea of reducing to simple experimentalexamination a class of phenomena, long known only asa subject of admiration or frivolous speculation, andcarried that design into execution in the most perfectmanner: whilst, again, the same scrutinising pene-tration has shown itself in detecting and measuring, withthe nicest accuracy, certain minute phenomena fromwhich a physical affection of light was deduced, of akind not only antecedently unsupported by any sort ofanalogy or probability, but even such as the mind canhardly attempt to grasp by anv satisfactory mode of