*6 5 
the Cause of Coloured Concentric Rings. 
transmitted through the base of the prism, and are reflected 
by the mirror in such a direction, as to come to the eye. I do 
not mention the blue bow streaks which may be seen with 
some attention ; they are very faint, and are not the object of 
this experiment, serving only to prove, that the coloured 
spectrum is formed in the bow place. 
Second experiment. Every thing remaining arranged as 
before, lay a narrow slip of thin pasteboard ef under the end 
of the prism at A C, figure 12, but leave B D in contact ; the 
eye must also be elevated till the bow place comes up to a b. 
In this position you will see the small image multiplied, so 
that, according to the brightness of the candle, and clearness 
of the prism and reflector, 6, 7, or 8 coloured spectra may 
be perceived, arranged from d tog as expressed in the figure. 
They are not perfect images of the candle, but so many re- 
flections of the red, yellow, and green light transmitted at 
the blue bow place, and every one of them will accordingly 
be seen to be nearly as broad in the green part, as in the red ; 
none of them coming to a tapering point, like the white image 
of the candle. The spectra are consequently occasioned by a 
reiterated reflection of the critically separated rays between, 
the subjacent mirror, and the exterior surface of the base of 
the prism. Indeed, nothing can be more evident than this 
reiteration of reflections, which is so well known, that oppo- 
site parallel mirrors are often put up in rooms, to produce a 
multiplied extent. 
Third experiment. When the eye is lifted high enough to 
have the line of the critical reflection quite above the prism, 
and the small screen is also taken away, we may repeat lifting 
and depressing the end A C, and the two reflecting surfaces 
Z 
MDCCCX. 
