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but be highly useful, as they may be depended on ; the interior 

 surveys of those parts having since proved that no erroneous inter- 

 sections had been made in those operations. 



The Bakerian Lecture. Experiments and Calculations relative to phy- 

 sical Optics. By Thomas Young, M.D. F.R.S. Read Novem- 

 ber 24, 1803. [Phil. Trans. 1804, p. 1.] 



It consists of six sections, the first of which is intended to convey 

 an experimental demonstration of the general law of the interference 

 of light. This demonstration rests on two experiments, the results 

 of which are brought in proof, that fringes of colours, and even the 

 crested fringes described by Grimaldi, are produced by the inter- 

 ference of two portions of light. These results are, that if one of 

 the two edges of a shadow produced by a narrow opake body be 

 intercepted by a screen at a small distance from that body, the op- 

 posite edge will no longer exhibit the fringed appearance which it 

 had in common with the former edge, when the latter was not inter- 

 cepted. 



Under the second head we have a comparison of measures of the 

 intervals of disappearance of light when refracted between two edges 

 of knives, or intercepted by a hair or a thin wire. The experiments, 

 which were partly suggested by some observations of Sir Isaac New- 

 ton, are here collected in tables : and the author states, as a general 

 inference, that if we thus examine the dimensions of the fringes 

 under different circumstances, we may calculate the differences of 

 the lengths of the paths of the portions of light which have been 

 proved to be concerned in producing those fringes ; and we shall 

 find, that where the lengths are equal, the light always remains 

 white ; but that where either the brightest light, or the light of any 

 given colour, disappears and reappears a first, a second, or a third 

 time, the differences of the paths of the two portions are nearly in 

 an arithmetical progression. 



In the third section, these principles are applied to explain the 

 repetition of colours sometimes observed within the common rain- 

 bow, particularly those described in the Philosophical Transactions 

 by Dr. Langwith and Mr. Daval. The train of reasoning here ad- 

 duced would lose too much of its evidence by being abridged. 



The fourth section is entitled, " Argumentative inference respect- 

 ing the Nature of Light." Here we meet with something of a con- 

 troversial nature, in which those who have adopted theories different 

 from that which our author is desirous to establish, are called upon 

 to explain his experiments according to their principles. What ap- 

 pears to him to operate chiefly against the advocates for the projectile 

 hypothesis of light, is, that light moves more slowly in a denser than 

 in a rarer medium, and that hence refraction is not the effect of an 

 attractive force directed to a denser medium. 



The fifth section treats of the colours of natural bodies. The na- 

 ture of the light transmitted by various bodies is here described, but 



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