as exhibited in its propagation along plates of glass . 73 
I next took a plate of glass that had a diamond cut across 
the middle, where the interior white fringes appeared. Having 
broken it in- two, a black fringe instantly arose between the cut 
mn % Fig. 14. (PL III.) and the plate displayed all the interior 
sets of fringes without receiving an additional supply of heat. 
I now attempted to bring the two separated surfaces into 
close contact by grinding them upon each other ; but I could 
not succeed in making them act upon light like a single plate; 
The following method, however, enabled me to surmount the 
difficulty, and to obtain some new results. 
I took a piece of annealed crown glass of the size repre- 
sented in Fig. 52. (PL V.) about 0.42 inches thick, and 0.5 broad, 
and having made a notch with a file at the point B, I applied to 
it a heated iron, which instantly produced a fissure Bb c d, and 
intercepted all the incident light by the total reflection which 
was produced. After standing an hour, this fissure began to 
disappear, and in the course of a day, it was as completely 
closed up as if it had never been made. The fissure was 
frequently reproduced by a hot iron ; and it regularly closed, 
unless when the expansive effect of the heat was capable of 
separating the surfaces to too great a distance. Sometimes it 
closed in a few seconds, and at other times a little mechanical 
pressure was requisite to effect the reunion. When the fissure 
was open, I laid the glass upon a hot iron, and it quickly pro- 
duced the fringes shown in Fig. 53. ( PL V.) where the pheno- 
mena are exactly the same as if the two pieces AB, BD, had 
been completely separated. But when the fissure was closed, 
and the glass laid upon the hot iron, it exhibited the different 
sets of fringes as shown in Fig. 54. (PL V.) just as if it had 
been one continuous mass* 
MDCCCXVI. 
L 
