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LV. Particulars of a Series of Experiments and Calculations 

 undertaken with a View to determine the Velocity with which 

 Light traverses Transparent Media, By R. Potter, Jun. 9 

 Esq* 



T>EFORE entering on the immediate subject of the essay, 

 " I must be excused a few remarks on Professor Airy's last 

 paper, in the Phil. Mag. for June (vol. ii. p. 451). He does not 

 inform us on what ground he came to the conclusion, that the 

 locus of interference for any one band after prismatic refrac- 

 tion was exactly in the theoretical direction. His adjusting the 

 wooden bar on which his eye-piece slided, so that a certain 

 interference band might be seen on the wire, is not sufficient, 

 unless the angle between the bar and the incident pencils is 

 known, together with other data needful in a calculation which 

 would require some nicety. This intricacy would have been 

 avoided by using a criterion furnished by the experiment itself, 

 such as the position of a band with respect to the diffracted 

 fringes given by the edges of one of the mirrors, being the 

 method which I employed. But Professor Airy does not in- 

 form us that he used any such expedient, nor that he found 

 an aplanatic lens needful for such experiments. 



To proceed to the subject of the paper; if ab, fig. 1, be 

 two luminous points from which luminiferous surfaces depart 



Fig. 1 



simultaneously, then the locus of the central band of inter- 

 ference will be always on the line p e, bisecting perpendicu- 

 larly the line joining a and b. In this we suppose the whole 

 path of the light to be in air. But if any other medium such 

 as a plate of glass with parallel surfaces were placed as at 

 g h i k, perpendicularly to p e, the place of central interfe- 

 rence would not be altered. If, however, another pate of the 



* Communicated by the Author, 



