as exhibited in its propagation along plates of glass. 6g 
Proposition XVI. 
To explain the effects produced upon the fringes by varying the 
height of the glass plates. 
In order to observe the changes occasioned by increasing 
the height of the plates, I employed pieces of glass whose 
height varied from 0.18 of an inch to 8 inches. When the 
height is very small, and not above 2 inches, the black spaces 
occupy nearly the position as shown in Fig. 2. (PL II.) The 
fringes are therefore very small, as they must always dimi- 
nish with the height, but they are remarkably brilliant, and 
exhibit much beauty in their developement. 
When the plates exceed two inches in height, the distances 
NP, PE, Fig. 2. ( PI. II. ) increase much faster than ND ; a 
smaller number of fringes is developed beyond OP, and their 
brightness is much impaired. (This effect is shown in Fig. 9. 
PI. II.) When the plates are 8 inches high, the whole 
fringe is faintly seen above OP. The colours of the two 
interior fringes are developed from a line much nearer MM 
than OP, and the black fringe OP is extremely indistinct. 
As high plates almost always burst in pieces when the maxi- 
mum tint is nearly produced, I was obliged to use plates of 
common window glass, but, on account of its dark green 
colour, I could not examine the phenomena with much satis- 
faction ; in using parcels of these large plates, much caution is 
necessary, as there is almost a certainty of some of them 
bursting with violence during every experiment. 
