on Mr. Potter's Experiment on Interference. 165 



Make a small wooden frame (like a window-frame, or 

 rather like the frame of a schoolboy's slate, but smaller) ; 

 with a saw cut through the centres of two opposite sides and 

 unite them by hinges ; keeping the parts in one plane, fix in 

 them a clear piece of glass with putty ; and when it is well 

 bedded, cut it with a diamond along the line of the hinges. 

 The frame will now turn well, carrying two pieces of glass 

 of the same thickness almost exactly in contact at the turning 

 line. Fix one side of the frame so that the glass in it re- 

 ceives, perpendicularly, the light which is to fall upon one of 

 the mirrors, and so that the line of the hinges divides the 

 pencils which fall upon the two mirrors. One pencil will then 

 pass through the fixed glass and one through the moveable 

 glass. Now if the glasses are in the same plane, the inter- 

 ference-fringes are not altered ; but if, while one is perpendi- 

 cular to its pencil, the other is inclined, the fringes imme- 

 diately shift towards the pencil which has passed through 

 the inclined glass. Thus far the experiment has often been 

 described; but the part which I wish particularly to point out 

 is the following : — If, after looking with the glasses in the same 

 plane, the observer should leave his eye-glass and change the 

 position of one glass and then return to his eye-glass, he 

 would find the centre of the fringes shifted, and might per- 

 haps infer that the fringes had shifted bodily, and that the 

 bar which was the central bar before is the central bar now. 

 Nothing could be less true; as he would see if by a little addi- 

 tional apparatus (a string, and a weight to act in opposition 

 to the string are quite sufficient,) he inclined the glass without 

 taking his eye from the eye-glass. The fringes shift, but at 

 the same time they change their character in such a manner 

 that till they have been observed a few times the eye is com- 

 pletely bewildered. Supposing (for clearness of ideas) that 

 the fringes shift to the right ; the central white bar as it travels 

 becomes blue at its right edge and red at its left edge, and 

 when it has shifted about four bars, the bright white bar is 

 the fifth from the original place of the bright white bar (I 

 do not pretend to great accuracy in these numbers). This ob- 

 servation, which is easily made without moving the eye, shows 

 clearly the difference between the shifting of the bars and the 

 shifting of the centre of the fringes. In Mr. Potter's experi- 

 ment, while (upon withdrawing the eye and eye-glass from 

 the prism) the centre of the fringes shifts, the bars themselves 

 (according to theory) remain nearly stationary ; but it would 

 not be easy to preserve that steadiness of eye which is neces- 

 sary for these observations. 



The other fact which Mr. Potter mentions, — namely, that 



