152 Mr. Potter on the Phenomena of Diffraction 



being 1', that of the maximum in the first bright ring is 1*16, 

 in the second is 1*09, and in the third is 1*06. 



To judge whether these or any of them could be seen, we 

 may compare them with the expression for the relative in- 

 tensities in Newton's rings seen by transmission, as given in 

 Mr. Airy's tracts, page 305. This expression gives the ratio 

 of the intensities in a dark and bright ring as 1* to 1*173, a 

 ratio of the same order of magnitude as the above, which Mr. 

 Airy assumes as representing relative intensities which pro- 

 bably could not be seen, although Newton's rings seen by 

 transmitted light are a well-known conspicuous phenomenon. 

 This also affects Mr. Airy's statement on a similar case in his 

 paper " On the Theoretical Explanation of an apparent new 

 Polarity in Light," in the Phil. Trans, for 1840, At p. 242, 

 he says, " The destruction of bands is here as to sense com- 

 plete," &c., where the ratio of the intensities in a minimum 

 and a maximum were as 1* to 1*287 in one case, and as 1* to 

 1*098 in another, and yet in the former the difference of in- 

 tensities is greater than in Newton's rings, which are admitted 

 to be distinguishable. In Mr. Airy's opinions and experience, 

 respecting the magnitudes which are sufficient to render lu- 

 minous points visible, we have to notice the following. In his 

 paper in the January Number of the Magazine, he says, " Sub- 

 stituting in the expression above, we find the diameter of the 

 bright spot = 0*000308 inch, or less than j^ inch, regard- 

 ing the visibility of which in common experience we need not 

 to disquiet ourselves. 



" If the diameter of the circular plate had been taken 1 inch, 

 the distance of the screen remaining the same, the diameter 

 of the spot would have been 0*0037 inch, a speck difficult 

 even for a philosopher to discover under these circumstances. 

 If the diameter of the plate were 0*1 inch, the diameter of the 

 spot would be 0*037 inch, a very fit subject for experimental 

 measure." In the fifth volume of the Cambridge Phil. Trans., 

 p. 110, we find Mr. Airy saying, " I have used both parallelo- 

 grammic and circular holes of different sizes(the largest circular 

 hole being \ inch in diameter), and have sometimes diminished 

 the aperture to as little as T o\j^ inch (by estimation.)" Thus 

 we have Mr. Airy estimating the magnitude of an aperture 

 of about roVo mcn at one time 5 anc ^ at another time he would 

 not have us disquiet ourselves as to the visibility of a bright 

 point surrounded by dark?iess, when its diameter was as little 

 as ^ _"__ inch. Yet we find the fixed stars are visible without 

 having any sensible diameter; and the images of Jupiter's sa- 

 tellites formed in a telescope, with an object-glass of 30 inches 

 focal length, have their diameters only about ^ono mcn ? all d 



