THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



NOVEMBER 1858. 



XXXV. On the Diffraction of Light. 

 By John Bridge, M.A.* 



IN the Philosophical Magazine for October 1855, 1 suggested 

 the application of photography to the illustration of the 

 diffraction of light. The attempts which I had then made were 

 sufficiently successful to warrant me in publishing that notice. 

 But renewing the experiments this summer, I have been able to 

 produce a very splendid series of phaenomena, which illustrate 

 the theory in every point. First, we draw on a large sheet of 

 paper a series of figures arranged at equal distances in a circle. 

 A collodion picture of these being taken, we have a series of 

 small transparent apertures in the elsewhere opake film. This 

 is then mounted so that each may be in turn brought before the 

 centre of a telescope, which is adjusted to view an image of the 

 sun. In this manner we have an apparatus of the most compact 

 form, by which a series of fifty or more different phaenomena 

 may be viewed in a few minutes. Also the figures being very 

 small (occupying on an average an area one-tenth of an inch in 

 diameter), the inaccuracies of surface and substance of the glass 

 may be neglectedf. There is another advantage in the figures 

 being small ; for the size of the image is in inverse proportion to 

 the size of the aperture. For this reason the apertures have 

 been made as small as was consistent with sufficient brightness. 

 I will now give a sketch of the several points which I have 

 sought to illustrate. In doing so I will endeavour to simplify 

 the mathematical treatment of such questions, and to explain 

 some new and curious phaenomena. 



* Communicated by the Author. 



t The film of Canuda balsam witli which a glass is cemented over the 

 picture, of course, produces no disturbance. 



Phil. Mag. S. 4. Vol. 16. No. 108. Nov. 1858. Y 



