NEWTON'S EXPERIMENTS ON DIFFRACTION, 109 



Then the proportion of the intensities of light along the horizontal 

 line in the first case will be the same as the proportion of the inten- 

 sities of light along a horizontal line in the second case: the distance 



x' = x y. -r in the second case corresponding to the distance x in the first 

 case. 



If in the first case the center of the hole is opposite to the center 

 of the slit, the horizontal line in the second case must be drawn over 

 the middle of the illumination on the screen. But if in the first case 

 the center of the hole is not opposite to the center of the slit, but 

 deviates in the direction which makes x positive, then the horizontal 

 line in the second case must not be drawn over the middle of the 

 illumination, but on that side on which y' is negative. In general, 

 or when one side of either aperture in the first case is wanting, the 

 equations 



may be used. 



When the inequality of the sides of the rhomboid is considerable, 

 the form of the illumination is not very different from the illumination 

 when the hole is parallelogrammic. The coloured bars will be a little 

 inclined, so that those which for a parallelogram would be perpendi- 

 cular to its longest sides, will approach towards the direction perpendi- 

 cular to the longer diagonal of the rhomboid. Besides these, there is 

 a faint brush of light projecting from each part which corresponds to 

 an obtuse angle, and nearly in the direction of a line bisecting that 

 angle produced. These general notions will assist the reader in judging 

 what ought, theoretically, to be expected in the different circumstances 

 of Newton's experiments. 



In Newton's experiments the external hole was in fact circular. 

 What would be the effect of this form it is impossible (theoretically) 

 to say: but judging from the insignificance of the effect produced by a 



Vol. V. Tart II. P 



