1 77 
exhibited by thin plates. 
produced by three slips of glass when laid together, and 
pressed by their own weight alone. Leaving them in this 
situation about fifteen minutes, I found an irregular set of 
binaries spread over the surface of those slips ; a representa- 
tion of which is shown in fig. 9. pi. IX. 
On drawing or sliding the upper piece of glass along the 
surface of the middle one, these fringes changed their shape, 
and disappeared as far as the middle slip was uncovered by 
the upper one ; or if the upper two were kept together, and 
both moved over the surface of the under slip, the same phe- 
nomena took place. But if the upper glass was in the smallest 
degree separated from the other two, or if the two upper 
pieces were in the same degree separated from the lower 
one ; in either case, the spectra first changed their shapes 
and then vanished ; but, on leaving the slips to the pressure 
of their own weight, were again as instantly restored. On 
changing the position of these three slips by placing the two 
upper ones across the lower one, a new spectrum was formed 
as seen in fig. 10, pi. IX. In these as well as in the following 
figures of binaries, the central band, dividing the two classes, is 
always denoted by a dotted line. 
These spectra may be varied almost to infinity by the 
smallest change of distance or relative position of any one of 
the three slips to the other two ; and it affords a pleasing 
amusement to observe those fleeting forms start into new and 
fantastic shapes in such a manner as strongly to resemble, in 
miniature, the coruscations of the aurora borealis. In order 
to determine whether these spectra were formed between 
the two upper, or between the two lower surfaces of the glass 
slips, the test of the shadow was applied, which determined 
A a 
MDCCCXV. 
