154 ^ r ' Herschel's Experiments for Investigating 
LV. Illustration of the Dependance of the Streaks of both the Bows, 
upon the critical Separation. 
One of the reasons, which in the 44th article have been 
given, for ascribing the colours of the bow-streaks to the cri- 
tical separation which causes the bows, is their being always 
in a direction parallel to the bows. With respect to this it 
rnay be thought by some, who are still inclined to believe in 
the fits of easy reflection, and easy transmission, that streaks 
parallel to the bows , ( though not dependant on critical separation) 
will in that situation be seen most easily, and most distinctly , 
because the visual ray in that position passes most obliquely through 
the stratum of air between the surfaces. This observation, how- 
ever, it will be found, cannot be applied to the streaks of either 
of the bows ; for in the 50th article it has already been 
proved, not that these streaks can most easily, and most dis- 
tinctly be seen in the place where the bows are, but that they 
can absolutely not be seen any where else. 
First illustration. To enter more minutely into a subject 
which is so essential to the support of the arguments con- 
tained in the 53d article, let us see whether it will be possible 
to assign any other reason why the streaks should be parallel 
to the bows, but their dependence on critical separation ? 
What is there in two plain surfaces that can determine the 
direction of streaks, supposing they could possibly be formed 
without depending on critical separation ? Why, for instance, 
should they not be as in figure 6, rather than in figure 7, 
since in both cases, their arrangement in the shape of a bow, 
would, according to the objection, be still in the position where 
the visual ray passes most obliquely through the stratum of 
