in Optical Instruments. 19 
Now, opticians have not been entirely insensible to the advantages, 
to be obtained by excluding all inefficient light ;—being aware that 
no kind of blacking applied to the inside of an optical tube, is 
sufficient to effect that salutary purpose, they have had recourse 
to other means, though inadequate to the end in view.—Thus it is 
common in a refracting astronomical telescope, to meet with one 
stop and sometimes two, placed in the interval between the object 
and eye-glass; the apertures of the magnifiers, are likewise con= 
tracted on the same principle. But these stops are never in suffi= 
cient number, or sufficiently contracted, or placed in such situa~ 
tions as they should be to be efficient, at least it has never fallen 
to my lot to see any such. It seems to me, as if they were pos= 
sessed of some superstitious dread of cutting off some of the light 
of the object glass by inserting stops; or perhaps have wished to 
shew their customers, that the apertures of their glasses were clear, 
it being a common trick to make a large object glass, and then to 
cut off the effect of the imperfect edges by a contrivance, such as 
has been mentioned, which ordinary purchasers are not aware of, 
and thus, suppose, the instrument to be much finer and better than 
it really is; at least it is not uncommon to meet with this species 
of fraud in the works of the continental artists, who are 
yery fond of making larger object glasses than the English work~ 
men. To enter into my subject, I shall here as succinctly as 
possible, describe the method which I have experimentally found 
to answer best for stifling fog in the astronomical refractor. It is 
a consideration which must obviously present itself, that if an eye= 
hole be placed at the end of a telescope, precisely of the size and 
precisely in the focus of the pencil of rays produced by any par 
ticular magnifier, that the end here proposed will be attained, as 
in the Gregorian and Cassagrain telescopes ; it will moreover, 
confine the eye truly to the axis of the tube, and thus prevent 
us from seeing any of that colour in the image, which may 
always be perceived in the best instruments, when the eye is a 
little removed from its true position. Nevertheless, I have after 
sufficient trial rejected this method, as less expedient than another 
which I shall point out, on account of the difficulty of executing it 
C 2 
