XXVII. A Theory of Luminous Rays on the Hypothesis of Undulations. By the 

 Rev. J. Challis, M.A., Plumian Professor of Astronomy and Experimental 

 Philo.wphy in the University of Cainbridge. 



[Read May 11, 1846.] 



If a beam of Sun-light pass through a narrow aperture, about one-thirtieth of an inch in breadth 

 and be received on a glass prism the edges of which are parallel to the borders of the aperture, 

 a spectrum is formed by the transmitted light, which, when magnified and properly looked at, 

 exhibits, as is well t;nown, a large number of dark lines parallel to the refracting edge of the 

 prism. If instead of passing through an aperture with parallel borders, the light passed through 

 a circular aperture, one-thirtieth of an inch in diameter, a spectrum of diminished width would 

 be seen, but of the same length as before and crossed by the same dark lines. The trans- 

 mission through the prism has produced no change on the light : it has only brought into view 

 the parts of which the incident beam is composed. Taking, for instance, a portion of light 

 immediately contiguous to any one of the dark lines, the prism informs us that the incident 

 beam contains light of that particular refrangibility, abruptly terminated in a plane passing through 

 the axis of the beam perpendicular to the edge of the prism. The existence of this abrupt ter- 

 mination is owing to the cause, whatever it may be, which produces the dark line, and has nothing 

 whatever to do with the transmission of the light through a small aperture. Let now the prism 

 be turned about the axis of the beam to any other position. The spectrum will present exactly 

 the same appearance as before, and light of the same refrangibility (not necessarily the same light) 

 as in the former case, will still be bounded by a dark line. And so for every position the prism 

 be made to take by being turned about the axis of the incident beam. This experiment proves 

 that every beam of white light contains portions of light of a definite refrangibility, the sides 

 of which are turned in all directions from the axis of the beam. Tiiis fact is at once explained 

 by supposing light to consist of rays; and it does not appear possible to give any other explanation 

 of it. Although the experimental evidence applies immediately only to the portions of light con- 

 tiguous to dark lines, yet a very strong presumption is afforded by it tliat all light is in the 

 form of rays. The existence of the dark lines themselves is most simply accounted for by 

 supposing that certain rays of Sun-light are in some manner extinguished. 



Admitting it to be a legitimate deduction from the facts of the ijolar Spectrum, that light is 

 composed of rays, it is clear that no Theory of Light can be complete which does not take account 

 of this distinctive character. The facts are perfectly consistent with the Theory of Emission, 

 and the advocates of that theory might justly appeal to them as evidence in its favour. My 

 object in this communication will be to shew that rays of light are also to be accounted for 

 on the Undulatory Theory. 



It must here be premised that it is not my intention to treat the Undulatory Theory, as 

 most optical writers of the present day have done, by a particular consideration of the molecular 

 constitution of the ffither. Not having been able to form the slightest concejjtion how this view of 

 Undulations can be reconciled with the existence of rays of light, I pro|)oso to regard the a'ther as a 

 continuous fluid sul)stance, sucli that small increments of its ])rcssure are proportional to small 

 increments of density, and to apjily to it the usual hydrodynamical equations. The pressure 



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