178 Mr. Knox on some phenomena of colours, 
them to be always formed between the two lower contiguous 
surfaces. 
Exp. 20. But it appearing very unaccountable that the 
mere presence of the upper glass slip should produce spectra 
between the two lower slips ; I wished to have the fact cor- 
roborated by another experiment. For this purpose two 
similar slips of glass were so closely applied together, as to 
produce primary fringes between them ; and in this position 
they were cemented together with bees’ wax, to prevent, 
shifting : this double slip being substituted, instead of the 
two upper unconnected slips in Experiment 12, produced 
binaries as usual. On application of the shadow, the prima- 
ries appeared in the second, and the binaries in the third 
shadow, as expected ; but it being perfectly certain that the 
primaries were formed between the two upper contiguous 
surfaces, and as the shadow proved that the thickness of 
one slip of glass was interposed between the two spectra, 
there could not remain a doubt of the binaries being formed 
between the two lower contiguous surfaces. 
Exp. 21. These results being so unexpected with three 
slips, it naturally occurred to me, to try what effect four would 
produce. Having placed two, laid together horizontally on a 
table, I took up the double slip mentioned in Experiment 18, 
in order to clear it of dust, and was in the act of holding it 
up between the light and the eye, in order to examine 
whether it was perfectly clean ; and it happening accidentally 
to be in the direction of the two lying, on the table, I was 
agreeably surprised to find a spectrum already appear, as if 
formed in the air, although the two pairs of slips were 
several inches asunder; which spectrum, on moving the 
