Photographic Impressions of Polarised Rings. 113 



It appears then that all the leading features of the abnormal 

 rings are perfectly explicable as a result of the superposition of 

 separately regular systems. But if known causes suffice for the 

 explanation of phenomena, we must by no means resort to agents 

 whose existence is purely hypothetical, such for example as in- 

 visible rays accompanying, but distinct from, visible rays of the 

 same refrangibility. Some of the minor details of the abnormal 

 rings may require further explanation or more precise calculation ; 

 but such calculations are of no particular interest unless the 

 phenomena offered grounds for suspecting the agency of 

 hitherto unrecognized causes. 



The difference between the photographs taken with iodide 

 and bromide of silver is easily explained, when we consider the 

 manner in which those substances are respectively affected by 

 the rays of the spectrum. With iodide of silver there is such a 

 concentration of photographic power extending from about the 

 fixed line G of Fraunhofer to a little beyond H, that even when 

 white light is employed we may approximately consider that we 

 are dealing with homogeneous rays. On this account, and not 

 because the rays of high refrangibility are capable of producing 

 a more extended system of rings than those of low refrangibility, 

 the rings visible on the photograph are much more numerous 

 than those seen directly by the eye with the same white light. 

 Moreover, the rings do not exhibit the same abnormal character 

 as with bromide of silver, in relation to which substance the 

 photographic power of the rays is more diffused over the spec- 

 trum. 



It is not possible to place the eye and a sensitive plate pre- 

 pared with bromide of silver under the same circumstances with 

 regard to the formation of abnormal rings. It would be easy, 

 theoretically at least, to place the eye and the plate in the same 

 circumstances as regards rings, by using homogeneous light; 

 but then, I feel no doubt, the rings visible on the plate would 

 be as regular as those seen by the eye. On the other hand, if 

 differences of colour exist in the figure viewed by the eye, they 

 inevitably arrest the attention, and it is impossible to get rid of 

 them without at the same time rendering the light so nearly 

 homogeneous that on that account nothing abnormal would be 

 shown. Hence Mr. Crookes's abnormal rings afford a very 

 curious example of the creation, so to speak, by photography of 

 forms which do not exist in the object as viewed by the eye. 



Pembroke College, Cambridge, 

 July 8, 1853. 



Phil. Mag. S. 4, Vol. 6. No. 37. Aug. 1853. 



