278 Dr. Herschei/s. Experiments for investigating 
the base into the side AC of the prism ABC fig 6, he will see 
an erect blue bow by two reflections, only one of which how- 
ever is the cause of the critical separation of the coloured rays, 
the other being a common one. For when a mean, refrangible 
blue-bow-ray falls with an obliquity ab C of 82° 17' 31" on 
BC, it will by refraction give the angle Bed = 85° 2' 56", 8, 
from which we obtain Ade — 49 0 57' 3", 3, which being the 
mean angle of the critically separated rays, they will by re- 
flection pass to the side A C, where the angle of the common 
reflection C ef will be 40° 2' 56", 8 ; this gives efB 85° 2 ' 56'', 8* 
and by refraction the middle of the blue bow will be seen by 
an eye at E in an angle JLg B equal to the angle ab C. From 
the construction of the figure, it is evident that the eye may 
be drawn from E towards a, and always keep the blue bow in 
view, which will still remain erect ; for when the eye comes 
to a, the rays by which the bow is seen will then enter at E, 
and the critical reflection will still remain at d, as may be 
satisfactorily proved by an application of the plain glass to AC, 
which will cause no streaks, whereas they will immediately 
appear when it is held under the side A B. 
When the eye looks into the side B C with the same obli- 
quity of 82° 17' 31", but differently directed, so that in fig. 7 
the angle may be a b B, instead of a b C a blue bow will again 
be seen, but in an inverted position. This also may be drawn 
over into the other side of the prism without an alteration of 
its appearance, the reason of which is sufficiently evident from 
the construction of the figure ; but in this case the critical re- 
flection will be a e, and the common one at d. 
It will be proper to shew that like appearances of the red 
bow may be seen ; for this purpose let the prism be laid with 
