152 EXPEBIMENTS AND INVESTIGATIONS 



dark intervals in the case of the hair, as they must do if the 

 rays moved in straight lines. 



But the case of the fringes or images which we have been 

 examining and reasoning tipon is wholly different. I have 

 measured the breadths of those formed by disposition and 

 polarization, and found that they are broad in proportion to 

 the distance from the bending edge of the chart on which 

 they are received ; and vary from the results given by similar 

 triangles in so trifling a degree, that it can arise only from 

 error in measurement. Thus in an average of five trials, at 

 the relative distances of 41 and 73 inches, the disc was Of at 

 the shorter, and 10^ at the longer distance ; the fringe next it 

 3/5 at the shorter, and 5-f T at the longer distance, whereas 

 the proportions by similar triangles would have been 9-J and 

 o, so that the difference is small, and is by excess, and not, 

 as in the hair experiment, by defect. Had the difference 

 been as in Sir I. NEWTON'S experiment, instead of 1 0-f and 6/3-, 

 it would have 3 ' T and l|f . In another measurement at 101 

 and 158 inches respectively, the disc was 15^, the fringe 8| 

 instead of 14f and 9-^ respectively. But by Sir I. NEWTON'S 

 proportions these should have been 34-f- and -^T- -^ i s plain- 

 that if the measures had been taken with the micrometer 

 instruments, which had not been then furnished, there would 

 have been no deviation. I have since tried the experiment, 

 not as above, on the fringes formed by the double-edged 

 instrument, but on those formed by one edge at a distance 

 behind the other, and have found no reason to doubt that the 

 rays follow a rectilinear course. 



It may further be observed, that in the fringes or images 

 by disposition and polarization, the dark intervals disappear 

 at short distances from the point of flexion, and that the 

 fringes run into one another, so that we find the red mixed 

 with the blue and violet. This is one reason why I often 

 experimented with the prismatic rays. 



3. It follows from the property of light, which I have 

 termed disposition, on one side the ray, and polarization on 

 the opposite side, superinduced by flexion, that those two 



