26 Dr. Goring on False Light 
as follows: the tube which carries the magnifiers is seven inches 
long, instead of being only two inches or perhaps less, as is usual 
—the diagonal metal is likewise placed nearer than usual to the 
large one, so that the length of the telescope is reduced about five 
inches; this is of course necessary to render the pencil of rays re= 
flected at right angles from the axis, long enough to act with the 
increased length of the eye-tube. (As the diameter of the spec- 
trum of the great mirror increases as we recede from its focal ex- 
tremity, more of the small plain one will in this case be called upon 
to act ; it however will still do the work without any increase in its 
size; mine is 1-3, inch of circular diameter, yet its entire surface is 
not employed.) By this arrangement a sufficient length of eye- 
tube is obtained to insert the stops 5, 6, and 7, as in the refractor, 
No. 5, is the efficient stop as before and is ;7; inch in diameter, 
The extrusion of the aberrant light is complete as long as the eye is 
in the axis of the instrument. 
It would of course be impossible to insert any stops similar to 
those, 1, 2, 3, and 4, in the refractor to render the effect more 
complete; nevertheless, I think, were it any object, a Newtonian 
would by the aid of the contrivance I have applied to mine, act 
sufficiently well with a skeleton tube only. It now remains for me 
to describe the new adjustment which the adoption of so long an 
eye-tube has compelled me to have recourse to, for it is evident 
that any want of centricity and parallelism in the lenses composing 
the eye-glass to the axis, which might be tolerated in a very short 
tube, will be perfectly insupportable when aggravated by a longer 
one: moreover, the stop No. 5, (which I suppose to be so ad- 
justed as barely to suffer all the light of the great metal to clear it) 
will, if not truly concentrical with the cone of light on which it 
operates, evidently impede some, as in such a case is perceptible 
by looking through the small eye-hole recommended in adjusting 
the refractor, or by examining the extreme pencil after it has passed 
the eye-glass with a magnifier. It is evident, I think, that the 
adjustment of a Newtonian is complete, when the pencil of rays 
which is reflected from the small metal, truly perforates the axis 
of the eye-tube, and the centres of the lenses composing it; it mat~ 
