UPON THE PROPERTIES OF LIGHT. 
253 
furnished with a similar curve surface the form is more complete, as in c d. But the 
straight edge being used after the first flexion of the curved one, clearly shows that 
the first edge bends as well as the second, indeed more than the second, for the side 
of the figure answ'ering to that curved edge is most curved. Fourthly, the whole 
experiments with twm edges directly opposite each other negative the idea of there 
being no inflexion; indeed they seem to prove the inflexion equal to the deflexion. 
The phenomena under Proposition X. can in no way be reconciled to the supposi- 
tion of the first edge not inflecting the rays*. 
2. We must ever keep in view the difference between the fringes or images de- 
scribed by Sir I. Newton and measured by him, as made by the rays passing on each 
side of a hair, and the fringes or images which are made without the interference of 
rays passing on both sides. It is clear that the rays which form those fringes with 
their dark intervals do not proceed after passing the hair in straight lines. Sir 1. 
Newton’s measures-f' prove this; for at half a foot from the hair he found the first 
fringe xTotti of an inch broad, and the second fringe ; and at nine feet distance 
the former were the latter instead of being ^ and i-q, and the latter less than 
and so of all the other measures in the table, each being invariably about one- 
third what it ought to be if the rays moved in straight lines ; and this also explains 
why the fringes do not run into one another, or encroach on the 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 
upon is wholly different. I have measured the breadths of those formed by disposi- 
tion 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 6f at the shorter, and 10^ at the longer distance; the fringe 
next it 3-1^ at the shorter, and at the longer distance, whereas the proportions by 
similar triangles would have been 9^ and 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 10^ and it would have been 3^ and 
In another measurement at 101 and 158 inches respectively, the disc was 15^, the 
fringe 8^ instead of 14f and respectively. But by Sir I. Newton’s proportions 
these should have been 4f and 3^. It is 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. 
* If you hold a body between the eye and a light, as that of a candle, and approach it to the rays, you see 
the flame drawn towards the body ; and a beginning of images or fringes is perceived on that side. 
t Optics, B. iii. obs. 3. 
