on the Affections and Properties of Light. 367 
and they always lay at right angles to that direction in which 
the grease was disposed by drawing the hand along it. 
Observation '2. Besides these polished hairs, many specula 
have fewer or more small specks and threads, rough and black. 
Perhaps every polished surface is studded with a number of 
small ones, invisible to the naked eye from the quantity of re- 
gular light which it reflects. I took, from a reflecting telescope, a 
small concave speculum not very well finished; its surface shewed 
several specks to the naked eye, and many with a microscope. 
Its diameter was of an inch, its focal distance two inches, 
and the sphere to which it was ground eight inches diameter. 
I placed it at right angles to the rays of the sun, coming through 
a small hole of an inch diameter, into a very well darkened 
room; I then moved it vertically, so that the rays might be re- 
flected to a chart 12 inches from the speculum, and consequently 
1 o from the focus : and though the focus appeared white 
and bright, yet on the chart the broad image was very diffe- 
rent. It was mottled with a vast number of dark spots ; these 
were of two sorts chiefly, circular and oblong. Of the former 
a considerable number were distinct and large, the rest smaller 
and more confused, but so numerous that they seemed to fill 
the whole image. None were quite black, but rather of a bluish 
grey, and the oblong ones had a line of faint light in the middle, 
just as is the case in shadows of small bodies. But the chief 
thing which I remarked was the colours. Each oblong and 
round spot was bordered by a gleam of white, and several co- 
loured fringes separated by small dark spaces. The fringes 
were exactly like those surrounding the shadows of bodies, of 
the same shape with the dark space, having the colours in the 
order, red on the outside, blue or violet in the inside ; the in- 
