18 Dr. Goring on False Light 
well as to feel the utility of removing it, I shall here give a slight 
account of the indistinctness occasioned by fog, (as it is technically 
termed) to which I propose to apply a remedy. Thus, when we 
look through a telescope admitting false light at a printed bill, the 
plate of a clock, or other such object, especially if the day is 
clear, and the sun shines on it, we find, (however perfect the in- 
strument may be in other respects,) the letters or figures do not 
appear nearly so black and sharp, as they will when viewed by the 
naked eye under the same angle, but rather of a brownish colour; 
in other words, the effect upon the eye, is similar to that of looking 
through a mist, or through glasses dimmed by moisture ; in short, 
what an ordinary observer would express by saying, the instrument 
, did not shew objects clear and distinct*. Now, on examining the 
pencil of rays proceeding from the eye-piece of such a telescope, 
with a magnifier, it will (supposing no other source of indistinct- 
ness exists,) be found surrounded by a variety of foreign rays, 
forming different halyos about it, instead of appearing like a span- 
gle on a piece of black cloth, which it will do, when all the false 
light is stifled, as in Gregorian and Cassagrain reflectors by their 
eye-hole, and in refractors with erecting eye-pieces, which have 
a stop between the two bottom glasses, producing the same effect, 
by suffering nothing but the true and genuine pencil of light, from 
the object glass or metal, to teach the retina, Indeed, in these in- 
“struments the quantity of spurious rays would be so great, as ab- 
solutely to preclude any thing like distinct vision, without the 
stops and eye-holes in question, In those to which I propose to 
apply an equivalent contrivance for extinguishing fog, though there 
may not be the same imperious necessity for its application; still 
I think the advantage to be gained by the improvement, is not to 
be despised, but will rather be admitted to be highly useful and 
appropriate, as placing optical instruments one step nearer perfec- 
tion than they would otherwise be, by producing the maximum of 
distinctness and clearness of vision, of which they can be rendered 
susceptible, consistently with their excellence in other respects.— 
» * All cbjects are of course equally affected by the fog, but it is more striking 
in those I have designated than in others, 
