178 Mr. Potter on a new Photometer by Comparison, and 



The great difference in intensity between the dark and the 

 bright rings which we here find, is certainly not to be ac- 

 counted for on any principles of interference yet proposed ; 

 and it furnishes a very strong argument against the undula- 

 tory theory, in which the effects of interference are supposed 

 to be perfectly determinate when we know the circumstances 

 of the interfering pencils. 



Dr. Young and Sir John Herschel have each given for- 

 mulae for this difference of intensity, which, it is important to 

 know, nearly coincide; the slight difference between them 

 arising only from the latter having introduced certain approxi- 

 mations to simplify the expressions he used. 



Sir John Herschel has deduced his formula from the rules 

 laid down by the late M. Fresnel ; and he finds that the mini- 

 mum of the light in the dark ring should be represented by 

 the expression !• — ^a, when the maximum of the light in the 

 bright one is represented by 1 •, and a is equal to the first re- 

 flection, and the light incident equal to unity. 



Taking the value of a =3?oth, we have 1— -^^= !• — "13 =*86 



and -86: !•:: 1-: 1-1538 

 or the intensity of the light in the bright rings should be to 

 that in the dark ones, as I '1538 to T, a result widely different 

 from 3*5 to 1*, as we have found by experiment. 



The great effect which we find to be produced by the in- 

 terference of a small portion of light, must be deducible from 

 any theory which is proposed as representing the true law in 

 nature ; and as a determined fact, it refutes an argument which 

 M. Fresnel advanced against the hypothesis, that the fringes 

 produced by the edges of bodies placed in a pencil of light 

 diverging from a luminous point, are caused by the inter- 

 ference of light which has suffered an evanescent reflection 

 with that which has arrived directly from the luminous point. 



I have applied the photometer also, to repeating M. Arago's 

 experiment, by which he has demonstrated that if the reflected 

 and transmitted rings could be superposed they would exhibit 

 a sensibly uniform light. 1 find this to be undoubtedly the 

 fact; and the experiment furnishes an excellent means of 

 trying the suitableness of the weather for using the photome- 

 ter, and also the fitness of the locality where we purpose to 

 experiment. 



When I had completed the photometer, I found it very 

 readily applicable to measuring the reflective powers of sub- 

 stances, of which we could never expect to procure suflicient 

 extent of surface to render the method of photometry by lamps 

 available : it requires only a very small extent of plane sur- 



