5 
relative to physical Optics. 
the shadow, in its way towards the fringes. Nor was it for want 
of a sufficient intensity of light, that one of the two portions 
was incapable of producing the fringes alone ; for, when they 
were both uninterrupted, the lines appeared, even if the intensity 
was reduced to one-tenth or one-twentieth. 
Exper. 2. The crested fringes described by the ingenious and 
accurate Grimaldi, afford an elegant variation of the preceding 
experiment, and an interesting example of a calculation grounded 
on it. When a shadow is formed by an object which has a rect- 
angular termination, besides the usual external fringes, there 
are two or three alternations of colours, beginning from the line 
which bisects the angle, disposed on each side of it, in curves, 
which are convex towards the bisecting line, and which con- 
verge in some degree towards it, as they become more remote 
from the angular point. These fringes are also the joint effect 
of the light which is inflected directly towards the shadow, from 
each of the two outlines of the object. For, if a screen be placed 
within a few inches of the object, so as to receive only one of 
the edges of the shadow, the whole of the fringes disappear. If, 
on the contrary, the rectangular point of the screen be opposed 
to the point of the shadow, so as barely to receive the angle 
of the shadow on its extremity, the fringes will remain un^ 
disturbed. 
II. COMPARISON OF MEASURES, DEDUCED FROM VARIOUS 
EXPERIMENTS. 
If we now proceed to examine the dimensions of the fringes, 
under different circumstances, we may calculate the differences 
of the lengths of the paths described by the portions of light, 
which have thus been proved to be concerned in producing those 
B 2 
