20 Dr. Goring on False Light 
properly with high powers, as well as that it confines the field of 
view, and is disagreeable to the eye. It is evident, that with high 
powers the pencils of rays will be exceedingly small, therefore if 
the aperture of the eye-hole is too large it will be ineffectual, if too 
small it will obstruct light : it must therefore be executed to a very 
great nicety, which is not always to be expected; besides in a 
case of such delicacy, if the eye-piece be not screwed on to a par- 
ticular mark on the body of the tube, or if any of the parts of 
which the magnifying apparatus is composed, be more or less 
screwed home than at the time of adjustment, it will be highly 
probable, that the eye-hole will be a little thrown out of its true 
situation, and thus do away with the sole object for which it was 
constructed. Thus it is, that what is perfect. in theory, will not 
always answer in practice. As to using eye glasses with very 
small apertures for the same purpose, (as contradistinguished from 
eye-holes, placed at the ends of the cones of light, drawn toa 
point by the magnifiers,) it is a method which cannot be made to 
exclude false rays with any degree of precision, even though their 
diameters are so much reduced as greatly to contract the field of 
view. I shall now give an account of the plan I have selected as 
most eligible, and which I have applied to a thirty inch and eigh- 
teen inch refractor with complete success, as it seems to me. Fig. 1. 
Plate II. isa drawing of the section ofarefractor, in which may beseen 
seven stops in the course of the tube and eye-piece, (exclusive of 
the field bar,) five of which are placed in such a manner, and of 
such apertures as to pinch the cone of rays proceeding from the 
object glass as tight. as possible, without intercepting any. It will 
be obvious that no foreign rays, or any that are not parallel, wilt 
be able to find their way to the eye, nor can any light be re- 
flected from the sides of the tube, so as to become visible. To 
execute this, use the following method; when the telescope is 
finished in the usual way, and before any stop is inserted, 
attach its lowest astronomical eye-piece to it, and find the true 
solar or sidereal focus of the object glass; when thus adjusted, 
measure carefully with a dynameter, the size of the pencil of rays 
proceeding frora the eye-piece, and note it down. Having then 
i 
