exhibited by thin plates. 1 7^ 
A representation is shown in fig. 5, ( pi. VIII.) before referred 
to, where the sets denoted by 1, 2, 3, and 4, may be called pri- 
maries in respect of 5, 6 , 7, and 8, which, in like manner, may 
be denominated transmitted sets ; all the space below the line 
CD being supposed covered with a shadow of a black card. • 
Not having hitherto observed these secondary fringes, ex- 
cept where they proceeded from intersections of primaries, 
it was concluded, that this was the only cause of their appear- 
ance ; but, by this last experiment, I was convinced of the 
error of this opinion, and that they are entitled to the same 
rank of originality as primary fringes or Newtonian rings ; 
and, since they are always found divided into tzvo classes, I 
shall henceforward venture to denominate them binaries, which 
perhaps may not be an improper characteristic for the whole 
genus, in contradistinction to primaries , as it will appear in 
the sequel, that there are several species of the former, and 
at least two of the latter. 
Exp. 16. This experiment was made to ascertain whether 
tw'o slips of glass only, when the uppermost was pressed by its 
own weight alone, would produce binary fringes between 
them ; and I succeeded in perceiving one set whose breadths 
were about one-tenth of an inch each, see fig. 9, (pi. IX.); 
these are best seen by clear day light ; but it is also necessary 
to have a piece of black velvet under the lower slip, otherwise 
to have its under side painted black. 
Exp. 17. In endeavouring to see these fringes mentioned in 
the last article by candle light, I was disappointed, but accident- 
ally perceived others of a different species, less in breadth, but 
much more numerous, and by which the whole length and 
breadth of the glass slips were covered. These were visible 
