8 REPORT — 1844. 



Oh the Propagation of Waves in a Resisted Medium, with a neiv Explana- 

 tion of the Dispersion and Absorption of Light, and other Optical Pheno- 

 mena.. By the Rev. M. O'Brien. 



The author notices two different hypotheses which may be made respecting the 

 mode of action of the particles of a transparent medium on the vibrations of the 

 ethereal fluid within it: — the iirst, " that the transparent substance exerts upon each 

 element of the ethereal fluid forces which depend simply upon the displacements of 

 that element relatively to the contiguous particles of matter :" this will be so, when 

 the amplitudes of the vibrations or maximum excursions of the elements from their 

 positions of equilibrium are extremely small relative to the intervals between the 

 particles of the transparent substance:- — the second, " that the forces exerted by the 

 transparent substance upon any element of the ethereal fluid are of the same nature 

 as the resistances experienced by a set of particles moving through a resisting medium, 

 depending not upon the relative displacements, but upon the state of motion of the 

 element;" this will be the case when the amplitudes of the vibrations are large, com- 

 pared with the intervals between the particles of the transparent substance. The 

 author then proceeds to show that M. Cauchy's equations are founded upon assump- 

 tions equivalent to the first of these hypotheses ; and gives reasons for not admitting 

 it, stating that though the explanation that author derives of dispersion is satis- 

 factory, the explanation of absorption is really fallacious. He then proceeds to exa- 

 mine, mathematically, the consequences of the second hypothesis, which he conceives 

 has not yet been taken up by any writer upon physical optics, and proceeds to show 

 the probability that it may be of much service in advancing the undulatorv theory of 

 light. 



Account of a new Proportional Compass. Bij Oliver Byrne. 



By a vernier at the centre, and a means of adjusting a series of points, this instru- 

 ment enables an observer, by the aid of tables, to multiply, divide, and compare lines, 

 surfaces, solids and angles, with considerable precision. 



On the Shape of the Teeth of the Wheels of the Clock in the New Royal Ex- 

 change. By E. J. Dent, F.R.A.S. 



A Notice explaining the Cause of an Optical Phenomenon observed by the 

 Rev. W. Selwyu. By Sir David Brewster, F.R.S.L.^E., Hoti. M.R.LA. 



When a number of parallel black lines are intersected at right angles by other 

 hlack lines, so as to inclose a number of squares or rectangles, a white spot appears 

 at the intersections of all the lines. In order to discover the cause of this phseno- 

 menon. Sir David Brewster made the experiment with the broad opake bars of an 

 old-fashioned window opposed to the hght of the sky. Along all the bars he saw a 

 whitish nebulous light, which was the complementary or accidental colour of the 

 black bars seen simultaneously with the bars. The same luminosity was of course 

 seen of equal intensity along all the bars, but at the crossings the intensity of its 

 light was greatest, so as to produce the white spot already mentioned. Now this 

 spot did not arise from any increased effect at the intersections, but from a diminu- 

 tion of the complementary luminosity at all other parts of the intersecting lines. 

 This diminution of intensity arises from the action of the white squares or rectangles 

 upon the retina tending to diminish the sensibility of that membrane along the parts 

 corresponding to the black lines, and is always greatest by oblique vision. It is an 

 action analogous to that which takes place when a strip of paper laid upon a green 

 or any other coloured glass disappears when the eye is fixed upon a point an inch 

 or two distant from the paper. Hence the lutninous spots are brightest when not 

 seen directly. [The phenomenon thus explained was communicated to Sir David 

 Brewster by the Rev. W. Selwyn.] 



