POPULAR SCIENCE REVIEW. 
286 
trical pencils. These defects can be diminished (but not 
entirely removed together), by using an 
eye-piece consisting of two convex lenses, 
instead of a single eye-glass. The two 
forms of eye-piece most commonly used 
are exhibited in figs. 5 and 6. Fig. 5 is 
the negative (or Huyghens*) eye-piece, 
so called because a real image is formed behind the field-glass 
(the lens which lies nearest to the object- 
glass). Fig. 6 represents th q positive (or 
Earns den ; s) eye-piece, so called because 
the real image formed by the object-glass 
lies in front of the field-glass. The 
course of a slightly oblique pencil through 
either piece is exhibited in the figures. 
The lenses are generally plano-convex, the convexities being 
turned towards the object-glass in the negative eye-piece, and 
towards each other in the positive eye-piece ; but Coddington 
has proved that the best forms for the lenses of the negative 
eve-piece are those shown in fig. 5. The negative eye-piece 
being achromatic, is commonly used in all observations re- 
quiring distinct vision only, but as it is clearly unfit for obser- 
vations requiring micrometrical measurement, or reference to 
fixed lines at the focus of the object-glass, the positive eye- 
piece is used for these purposes. 
We may now proceed to apply the facts just detailed to the 
determination of just methods of examining, testing, and 
using an astronomical telescope. The excellence of the object- 
glass can only be satisfactorily determined by testing the per- 
formance of the telescope in the manner presently to be noted. 
But it is well for the intending purchaser to examine the 
quality of the glass as respects transparency and uniformity of 
texture. Bubbles, scratches, and other such defects, are not 
of importance, since they do not affect the distinctness of the 
field, as they would in a Galilean telescope, — a little light is 
lost, and that is all. The same remark applies to dust upon 
the glass. The glass should be kept as free as possible from 
dirt, dust, or damp, but it is not advisable to remove every 
speck that, despite such precaution,, may accidentally fall upon 
the object-glass. When it becomes necessary to clean the 
glass, it is to be noted that the substance used should be soft, 
perfectly dry, and free from dust, and that when a small space 
near the edge of the glass has been cleaned, the dust is to be 
swept off from that space as centre, not gathered up by pressure, 
as is commonly done. The two glasses should on no account 
be separated. 
Fig. 6. 
Fig. 5. 
