17b Mr. Knox on some phenomena of colours , 
by the light of a small candle only, nor could they be per- 
ceived except in the image of the blaze of the candle 
reflected from the upper surface of the upper slip of glass, 
and by moving either the eye, or the candle, or the slips 
across the direction in which the fringes appeared : and even 
with every precaution, I sometimes failed in seeing them, 
which I attributed to the presence of very minute dust between 
the glass plates, which in some cases can be known only by 
its effects. Moreover, when the eye changes from a greater 
to a less quantity of light, it requires some time to adapt itself 
to such delicate and minute objects, before it can perceive 
them satisfactorily. See fig. 8. (pi. IX.) 
Exp. 18. By subsequent trials it was found, by the use of 
an oil lamp, having five or six small wicks in a row, composed 
of hempen packthread, which produced a pale brownish light, 
that those singular fringes could be seen with nearly as much 
ease and certainty as any of the other phenomena, though not 
all at once; but, by moving the light over them, otherwise 
moving the slips with respect to the light, and by using a mag- 
nifying glass, it was ascertained that the specimen represented 
by fig. 8, (pi. IX.) contained thirty of these fringes to an inch. 
Being uncertain whether they might not be the same as the 
primary fringes mentioned in Experiment 11, or a continuation 
of them, I caused a few of these latter to be formed at one end 
of the slips at A, fig. 8, ( pi. IX. ) when it was found that they 
crossed each other at a certain angle as shown in the figure ; 
therefore, they could not be the same. Moreover, as they 
have no perceptible colour like first primaries, nor are divided 
into classes as binaries, they must be different from either. 
Exp. i 9. It next occurred to try what effect could be 
