1 66 Dr . Herschel's Experiments for Investigating 
will then, by reiteration in the common way, give us a set of 
complete tapering images of the white flame of the candle, 
arranged as in figure 13. And in consequence of the various 
distance of the prism from the mirror, the distance of the 
several images of the candle, will increase and decrease, so 
that when at last, the end A C is again set down on the 
mirror, they will apparently coalesce into one single bright 
image. 
The use that may be made of these experiments is as fol- 
lows. From the laws of reflection we know, that the extent 
of the multiplied images perceived in reiterated reflections, 
between two surfaces, may at all times be ascertained when 
their distance is given. It is also well known, that when two 
plates are in what is called contact, we can in fact, only sup- 
pose them to be extremely near each other. The production 
of streaks, when a plain glass is laid under a prism, is a suf- 
ficient proof that, even when they are in the closest contact 
which can be made, the subjacent surface still acts by reflection 
upon the rays that pass through the base of the prism ; for 
if the contact of the two surfaces were so complete as to 
make one solid mass of glass, no reflection would take place 
within its substance. 
LX. Of the breadth of the Streaks compared to that of the Bows, 
and the Cause why they i\mst take up a broader Space than the 
Bows from which they are derived. 
It must have been noticed by those who have examined the 
beautiful streaks, which in my paper it has been shown, will 
be produced when a plain surface is held under the base of 
the prism, that they take up a broader space than the bows ; 
