HOW TO WORK WITH THE TELESCOPE. 
287 
In examining an eye-piece, the quality of the glass should 
be noted, and care taken that both glasses (but especially the 
field-glass) are free from the least speck, scratch, or blemish 
of any kind, for these defects will be exhibited in a magnified 
state in the field of view. Hence the eye-pieces require to be 
as carefully preserved from damp and dust as the object-glass, 
and to be more frequently cleaned. 
The tube of the telescope should be light, but strong, and 
free from vibration. Its quality in the last respect can be 
tested by lightly striking it when mounted ; the sound given 
out should be dead or non-resonant. The inside of the 
tube must absorb extraneous light, and should therefore be 
coloured a dull black ; and stops of varying radius should be 
placed along its length with the same object; sliding tubes, 
rackwork, &c., should work closely, yet easily. As respects 
the mounting, I shall have something to say in another 
paper. 
But the mere examination of the glasses, tube, mounting, 
&c., is only the first step in the series of tests which should 
be applied to the instrument, since the excellence of a tele- 
scope depends, not on its size, the beauty of its mounting, or 
any extraneous circumstances, but on its performance. 
The observer should first determine whether the chromatic 
aberration is corrected. To ascertain this the telescope 
should be directed to the moon, or (better) to Jupiter, and 
accurately focussed for distinct vision. If, then, on moving 
the eye-piece towards the object-glass, a ring of purple 
appears round the margin of the object, and on moving the 
eye-glass in the contrary direction a ring of green, the 
chromatic aberration is corrected, since these are the colours 
of the secondary spectrum. Sir David Brewster remarks, that 
if the achromatism is imperfect, “ the defect may be easily 
removed by altering slightly the curvature of one or other of 
the lenses.” This is not an experiment I should advise any 
one to attempt, however, lest the remedy prove worse than 
the disease. 
To determine whether the spherical aberration is corrected, 
the telescope should be directed towards a star of the third or 
fourth magnitude, and focussed for distinct vision. A cap 
with an aperture of about one-half its diameter should then 
be placed over the object-glass. If no new adjustment is re- 
quired for distinct vision, the spherical aberration is corrected, 
since the mean focal length and the focal length of the 
marginal rays are equal. If when the cap is on, the eye- 
piece has to be pulled out for distinct vision, the spherical 
aberration has not been • fully corrected ; if the eye-piece 
has to be pushed in, the aberration has been over- corrected. 
