exhibited by thin plates. 179 
double slip in the hand, out of the direction of those on the 
table, vanished as far as those on the table were uncovered 
by those in the hand. And notwithstanding this spectrum 
appeared to be formed in the air, the test of the shadow 
proved it to be formed between the two slips on the table ; 
and although it was visible when the two pairs of slips were 
not less than ten inches asunder, it could not be perceived 
except through the medium of the double slip held in the 
hand. 
A representation of this experiment is shown in fig. 1 1 , 
pi. IX. where AB is a section of the two slips held in the hand ; 
and CD of those on the table. Fig. 12 pi. IX. represents one 
of those fleeting and fantastic forms as seen by the eye at 
F in this experiment. 
Exp. 22. If the result of the last experiment appears ex- 
traordinary and unaccountable, the present one will appear 
still more so ; for, on lowering the double slip in the hand 
from its former elevation of about 45 degrees, to about 15 
degrees, so that its image was seen reflected from the upper 
surface of the upper glass on the table, as represented by 
GH in fig. 13. pi. IX., a new spectrum, fig. 14, pi. IX. was 
seen in this reflected image, superadded to that shown in 
fig. 12, which was not to be perceived in the object A B itself ! 
Both spectra together, as seen by the eye at F, are repre- 
sented in fig. 15. 
Exp . 23. It was fully ascertained by the test of the shadow, 
that while the double slips were in the position described in 
Experiment 20, both spectra were formed between the two 
slips on the table ; but, on lowering the two upper slips until 
they were laid flat on those incumbent on the table, both 
A a 2 
