160 Dr. Herschei/s Experiments for Investigating 
avail myself of what has been proved in the 37th article ; for 
it has been shown, that irregular surfaces will cause irregular 
figures ; for which reason such appearances will only prove, 
not that plain surfaces can produce them, but that the sur- 
faces between which we see them, are not strictly plain. 
Second remark. If it should still farther be conceived that 
by means of a wedge-formed plate of air, strait bands of colour 
would be produced betwee?i plain surfaces slightly inclined , the 
following experiment will show, that the objection cannot be 
well founded. I selected two plates of glass, their surfaces 
being as perfectly plain and parallel as I could possibly find 
them, and the event shows that they were sufficiently so. 
The plates were applied to each other in such a manner, that 
the end of one touched the surface of the other, in a verv 
sharp straight line, while at the opposite end they were kept 
from contact, by a very fine single thread of the silk worm 
placed between them, which would produce the required 
slight inclination. No streaks were then visible. I pressed 
the line of contact strongly together, and streaks became vi- 
sible ; but they were disfigured by pressure, and most disfi- 
gured where I pressed most. As soon as the pressure was 
removed, these coloured appearances vanished. My plain 
slips were cut with a diamond out of a parallel plate of glass, 
polished by an optician for optical purposes, and the incumbent 
slip had its tangent edge finely ground in an angle of about 
70 or 80 degrees, to make it a straight line without injuring 
its plain figure. 
Third remark. It will be proper also to take notice of an 
objection that may be made to the foregoing experiment, by 
appealing to one of an opposite result ; for possibly two plates 
