126 Diffraction Experiments. 



and having made their journeys in precisely the same time ; 

 but undulations extending from F towards E -will meet other 

 undulations that started at the same time, but reached E 

 quicker than those that came round the other corner. These 

 will interfere and give rise to dark or coloured bands, according 

 as their interference is complete or only partial. Dr. Young 

 introduced the screen C D to cut off the undulations that passed 

 B, and by so doing he stopped the interferences, and no fringes 

 appeared. 



We have thus arrived at a general explanation of why dark 

 lines appear when white light is viewed through a very narrow 

 slit, cut in a piece of metal or card. 



If, instead of such a slit, we obtain a piece of glass ruled 

 with fine lines, a five hundredth or a thousandth of an inch 

 apart, the flame of a candle seen through them offers a splendid 

 spectacle. A glass of this description, ruled very beautifully 

 by Messrs. Home and Thornthwaite, is in our hands at this 

 moment ; we hold it near the eye, so that the lines are vertical, 

 and look at a candle-flame a few feet off, or at the sky in the 

 day time. The candle-flame is seen in its natural colour in the 

 centre, then on each side of it a dark space, and after that, in 

 succession, right and left, a number of images of the flame in 

 prismatic colours, the reds being always furthest from the 

 central flame, and the blues nearest to it. The first few of these 

 coloured images of the candle-flame are separated by considera- 

 ble though lessening dark spaces from each other, and they are 

 followed by numerous other images, .running together, and 

 forming a beautiful spindle-shaped pattern of variegated light. 

 If we turn the ruled glass round, so that the lines are horizontal 

 instead of vertical, the candle-flame gives us a continuous 

 spindle of light, in which the several images of the flame run 

 into each other. The optical effect is as if a number of coloured 

 candle-flames were all fixed to a sort of a windmill arm and 

 rotated about on a central axis, and at the same time each flame 

 rotated about its long axis, so that whereas, at the beginning of 

 the experiment, we saw its breadth and depth from top to 

 bottom, at the end we see little more than the tips of the 

 flames. 



The effect varies according to the distance of the candle- 

 flame from the ruled glass. Close to the flame the coloured 

 images are confused, near each other, and deficient in brilliancy. 

 As the spectator (carrying the ruled glass with him) recedes 

 from the flame, the coloured images grow brighter, and 

 separate from each other, and after they are thus separated, to a 

 certain extent, revolving the ruled glass ceases to change the 

 form of the pattern, as just described, and merely causes the 

 spindle of coloured light to revolve. Eight and twenty feet off 



