VII. Supplement to the first and second part of the paper of experiments, for investigating the cause of coloured con­centric rings between object glasses, and other appearances of a similar nature

When the intricacy of the subject, on which my two last papers have been treating, is considered, it will not appear singular that a few supplementary articles should be given. The compression of the account of the experiments into a small compass, where many material circumstances must be left unnoticed, may throw some obscurity on the results, which can only be removed by examining the subject in a fuller ex­tent, and from various points of view. I hope the following illustration and additional explanations will have the effect of clearing up what may possibly to some appear obscure or doubtful, in either the first or second part of my paper, and serve also to make the conclusions, which in the second have been chiefly supported by prismatic experiments, directly applicable to such as have in the first been made by convex glasses. That the colours in all prismatic phenomena, which have been examined in the 44th, 45th, 46th, 47th, and 48 articles of my paper, are produced either by the interior critical separa­tion arising from the different reflexibility of the rays which cause the blue bow, or by the exterior critical separation arising from the different intromissibility of the rays which cause the red bow, has been so clearly and circumstantially proved that it can admit of no doubt; it may even be con­ceived by some that I have been too particular in giving the precise angles, when we see in the LectionesOptica , Sect. II. Par . 2, page 257, 258, how far Sir I. Newton has explained the blue bow ; but a sufficient reason for this minuteness was to give greater clearness to my explanation of the new pheno­menon of a red bow, which I have with equal precision de­scribed, and which by this means may be, step by step, compared with the production of the blue bow; but a sufficient reason for this minuteness was to give greater clearness to my explanation of the new phenomenon of a red bow, which I have with equal precision described, and which by this means may be, step by step, compared with the production of the blue bow. By this precaution I hoped to anticipate any objection that might occur, such as, for instance, that Sir I. N ewton has also explained the red bow which (it may be supposed) merely the converse of the blue bow. This conception, although N ewton no where speaks of a red bow, seems to be countenanced by what is said after he has shown that the blue bow is caused by the different reflexibility of the rays of light; for as he affirms that the red, orange, and yellow colours are transmitted, he contrives a method of proving it experimentally, by adding a second prism, placed under that which gives the blue bow, and thus making the transmitted rays visible. The full import of this Newtonian experiment will be considered in the following articles.