refraction to glass, &c. by pressure. 175 
and formed a black space as in Fig. 21. (PI. X.) By con- 
tinuing the inclination, the black space opened, and gradually 
developed the black spaces, and the colours shown in Fig. 24, 
(PI. X.) the fringes between A and D, and B and C having 
ascended in the scale, while those between A and B and D 
and C had descended. At the end of other two days, three 
distinct orders of colours were developed ; but when the isin- 
glass had detached itself from the glass bottom of the trough , 
the tints again descended to the state in which they are 
represented in Fig. 24. (Pi. X.) 
This descent of the tints will be understood from Fig. 25, 
2 6, 27. (PL X.) In virtue of the capillary attraction of the sides 
of the trough AB, the fluid jelly rises up at the angles a, b> 
and being there speedily hardened from its thinness, it ad - 
heres firmly to the sides of the trough. As the process of indu- 
ration advances, the plate of isinglass is gradually detached 
from the glass bottom, at the corners m, n , Fig. 2 6, (PI. X.) 
but still adheres firmly at the middle c. Hence the isinglass is 
in a state of great distension between c and a, and c and b , 
and consequently developes several orders of colours. But 
when the isinglass separates from the glass bottom at c , which 
it almost always does, it takes the position shown in Fig. 27, 
(PI. X.) where the distension has obviously suffered a great 
diminution, and consequently the tints must descend in the 
scale. The adhesion at c is sometimes so strong that the 
isinglass carries up a portion of the glass along with it. 
The combined effect of induration and distension in a narrow 
glass trough is shown in Fig. 28, (PI. X.) which represents 
one half of the trough. The narrow fringe produced from 
induration is shown at A mn and B op, and the tints developed 
