and on some Phenomena connected therewith. 199 



to the inch, and with reasonably flat glass as support to the 

 photographic film, these bands rarely invade the first or second 

 spectrum. When, however, we come to 17,000 lines to the 

 inch, it requires pretty flat glass and some precautions in 

 printing to keep even the first spectrum free from them. 



It was obvious from the first that the formation of these 

 bands was a question of the distance between the ruled surface 

 of the original and the sensitive film ; but it is only within 

 the last year or so that I have submitted the point to special 

 experiment. For this purpose I substitute for plane-parallel 

 glass as a substratum for the sensitive film the convex surface 

 of a lens of moderate curvature. As in the experiment of 

 Newton's rings, we obtain in this way an interval gradually 

 increasing from the point of contact outwards, and thus 

 upon one plate secure a record of the effect upon the copy of 

 varying degrees of closeness. When a spectrum of any order 

 is thrown upon the eye, those places upon the grating where 

 the spectrum in question fails appear as dark rings. My first 

 experiment of this kind was made with the Rutherfurd grating, 

 in order principally to find out how close a contact was really 

 necessary for copying. From the diameter of the first dark 

 ring, in conjunction with a rough estimate of the curvature 

 of the lens, I concluded that the interval between the surfaces 

 should nowhere much exceed 10 q 00 of an inch. It appeared 

 at the same time that the chance was remote of obtaining 

 a satisfactory performance in the spectrum of the second order. 

 About this time the theoretical views occurred to me which 

 will presently be explained, and I purposed to check them 

 by more careful measurements than I had yet attempted. In 

 the course of last summer, however, I found accidentally that 

 Fox Talbot had made, many years ago*, some kindred ob- 

 servations ; and the perusal of his account of them induced me 

 to alter somewhat my proposed line of attack. It will be 

 convenient to quote here Fox Talbot's brief statement : — 



" About ten or twenty feet from the radiant point, I placed 

 in the path of the ray an equidistant grating f made by 

 Fraunhofer, with its lines vertical. I then viewed the light 

 which had passed through this grating with a lens of con- 

 siderable magnifying-power. The appearance was very 

 curious, being a regular alternation of numerous lines of bands 

 of red and green colour, having their directions parallel to the 

 lines of the grating. On removing the lens a little further 

 from the grating, the bands gradually changed their colours, 



* Phil. Mag. Dec. 183G. 



t A plate of glass covered with gold leaf, on which several hundred 

 parallel lines are cut, in order to transmit the light at equal intervals. 



