in Optical Instruments. 27 
ters not, I conceive, at what angle or in what direction the said 
cone of rays proceeding from the large metal be thrown by the diago- 
nal one, provided these conditions are fulfilled, (supposing of course 
the position of the small metal to be the centre of the tube, so that 
it shall truly receive the whole of the light of the great one.) 
We may, therefore, either adjust the small metal to the eye-tube or 
the eye-tube to the small metal, or we may do both, which latter 
will probably be the most expedient, and is the method I have pre- 
ferred; I have effected it in a very simple manner, by having the 
tube made to fit loosely into another wider piece, which is screwed 
on in the usual way, to the side of the telescope:—the vacancy be- 
tween them is filled up with wax, the inner tube is tight at the 
bottom of the external one, by the interposition of a small setting 
chamferred at the edge, but admits of a slight rotatory motion to- 
wards the eye-glass by heating the wax with the flame of a candle 
- which is inserted into the external tube, and which unites them 
both together; time will be given to adjust it before the wax cools, 
when it will all set tight, and will not be liable to get out of order. 
Two small niches should be made, one in the shoulder of the screw 
of the external tube, and the other in that of the female screw to 
which it is applied, to be a guide that the two pieces may always 
be serewed home to a particular point: or it is very probable the 
adjustment may be spoiled, because it will be a chance if the eye- 
tube when fixed, is precisely at right angles to the side of the tele 
scope. A variety of methods of effecting this adjustment will pre- 
sent themselves to the workman, instead of that which I have used, 
which though it answers very well, yet does not look very elegant 
or scientific. Thus, instead of the wax, three screws might be 
used, fixed into the external tube; or such a contrivance as is repre- 
sented in Fig. II., by having counter screws to play against those 
by which the setting for the eye-piece is attached to the rackwork, 
on the side of the telescope, &c. Now it is not my intention to 
assert that this adjustment is absolutely indispensable, for I haye 
not a doubt but that a superior workman might execute a long eye- 
tube, such as I have employed, so that nothing but the usual adjust- 
ments would be required; still I think that no Newtonian would be 
