364 Mr. Talbot*s Experifizent on the Interference of Light, 



as 305 (the weight of the first) is to 90 (the weight of the se- 

 cond) as 108 (the atomic weight of the silver) is to 31*8, instead 

 of 31*6 (the atomic weight of the copper). In like manner, 

 305; 92*5: : 108 (the atomic weight of the silver) is to 32*8 

 (the atomic weight of the zinc,) instead of 32*5, found by Mr. 

 Faraday. It is evident, then, that the apparatus with a con- 

 stant current, and consisting only of two pairs, together with 

 the electro-magnetic balance, enables us to find the atomic 

 weights of metals, and to determine the quantities of reduced 

 metal that correspond to a given intensity of current. 



LXXII. An Experiment on the Interference of Light, 

 By H. F. Talbot, Esq,, F.R.S.* 



T BELIEVE the following experiment to be a new one, and 

 ■*• it seems to alFord a satisfactory illustration of the theory. 



Make a circular hole in a piece of card of the size of the pupil 

 of the eye. Cover one half of this opening with an extremely 

 thin film of glass (probably mica would answer the purpose 

 as well, or better). Then view through this aperture a per- 

 fect spectrum formed by a prism of moderate dispersive power, 

 and the spectrum will appear covered throughout its length 

 with parallel obscure bands, resembling the absorptions pro- 

 duced by iodine vapour. 



The cause of this phaenomenon probably is, that one half 

 of the light which passes through the glass film his its undu- 

 lations thereby retarded by a certain quantity, which may be 

 called A. 



Let L be the length of the undulation of any coloured ray, 

 which I suppose to be a much smaller quantity than A. 



Then if we consider the colours in succession, L increases 

 progressively from the violet to the red. Consequently the 



quotient -|- becomes by turns a whole number and a fraction, 



and then again a whole number, and so on alternately a 



A 



great number of times. Whenever -p- is a whole number, 



the two halves of the light agree in the phase of their undu- 



lation. But when -y- is midway between two whole num- 



bers, the two portions of light are opposed in phase, and 

 therefore the corresponding colour cannot make its appear- 

 ance in the spectrum at all ; and therefore also a dark band 

 appears in the place it would have occupied. 

 • Communicated by the Author. 



