Prof. Airy's Repetitions of Mr. Potter's Experiment. 4-5 1 



first time that I witnessed the electric spark by magnetic ex- 

 citation, it was shown to me by Mr.Watkins, in his shop at 

 Charing- cross, some considerable time after it had been shown 

 in London by Mr. Saxton, with a similar apparatus. 



LXXVI. Results of the Repetition of Mr. Potter's Experiment 

 of interposing a Prism in the Path of Interfering Light, By 

 Professor Airy*. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 

 I" HAVE lately had several opportunities of repeating, under 

 *- favourable circumstances, Mr. Potter's experiment of in- 

 terposing a prism in the path of interfering light, and am able 

 to assert positively, as an experimental fact, that the gradual 

 displacement and ultimate disappearance of the centre of the 

 fringes take place in the manner which I stated as a conse- 

 quence of theory ; namely, that on receding from the prism, 

 the fringes remain stationary; while their character changes, in 

 such a manner, that the centre of fringes passes gradually and 

 rapidly from the centre of the mixture of lights to its border. 



The apparatus which I have used consists of an eye-piece, 

 with a wire fixed in its focus, attached to a support which 

 slides on a bar that is placed in a position parallel to the path 

 of the light after refraction at the prism. By proper adjust- 

 ment of this bar, the wire may be kept steady upon one of the 

 fringes while the eye-piece is drawn from contact with the 

 prism to the greatest distance at which the fringe is visible. In 

 this manner I have kept one fringe under the wire, with the 

 certainty that, though its colour has altered, it has not de- 

 viated half the breadth of a fringe ; while the centre of fringes 

 has gradually moved through the space occupied by twelve 

 double fringes. 



I have made the experiment with light of various degrees 

 of heterogeneity, and in all cases, as far as I could judge, the 

 displacement of the centre of fringes was the same at the same 

 distance ; which is also a result of theory. It is necessary to 

 observe that, when the light is nearly homogeneous, the num- 

 ber of visible fringes is so much increased that it is difficult to 

 fix precisely on the centre of fringes. The light was coloured 

 by the use of five different red glasses (one of which made 

 the light nearly homogeneous) and of one green glass. 



I am, Gentlemen, your obedient Servant, 



Observatory, Cambridge, May, 16 1833. G. B. AlllY. 



* Sec our Numbers for February, March, April and May.— Edit. 

 3 M 2 



