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focussed for one, this might he replaced by any other without 
necessitating any use of the focussing rackwork. This could 
be readily effected by suitably placing the shoulder which 
limits the insertion of the eye-piece. Theoretically, all celestial 
objects require the same focus. One of our most careful ama- 
teur observers remarks, however, that the moon requires a 
different focus than more distant objects. If there is any 
appreciable difference, the cause must be sought elsewhere than 
in the difference of distance, since it may readily be shown 
that the focal length for parallel rays exceeds the focal length 
for the moon's distance by less than one-650, 000th part. 
Some observers have noticed that for distinct vision of spots 
near the sun* s limb a shorter focus has to be used than for dis- 
tinct vision of stars and planets, or of spots near the centre of 
the solar disc. Mr. Dawes thinks this an optical deception. 
It is necessary in selecting “ powers " to remember that 
different objects will bear different powers, — the stars will bear 
higher powers than the moon, Venus, and Mercury, these 
than Jupiter or Saturn, and these again than comets and 
nebulse. In all cases it must be remembered that high 
powers increase the difficulty of observation, since they 
diminish the field of view, increase the rapidity with which 
(owing to the earth's motion) the image moves across the 
field, and magnify all defects due to instability of the stand, 
imperfection of the object-glass, or undulations of the atmo- 
sphere. A good object-glass of four inches aperture will, in 
very favourable weather, bear a power of about 400 when 
applied to the observation of close double, or multiple stars, 
but for all other observations much lower powers should be 
used. “ He who possesses a tool of this size," says Admiral 
Smyth, speaking of five-foot refractors, “ ought never to teaze 
it with the higher powers except under obvious necessity." 
It is, of course, useless to diminish power so much that 
the emergent pencils of rays are larger than the pupil of 
the eye. In a Galilean telescope (as may be seen in the case 
of an opera-glass), such low powers are employed, because 
in this way only a tolerably large field can be obtained but 
* My attention has been called to the opinion expressed by Sir W. 
Herschel, that a concave lens would give as large and distinct a field as a 
convex lens of the same size and power, if only the eye were moved about 
in front of the lens. Kitchener found this to be the case, but (as might be 
expected) vision much less pleasant with the concave lens. The difference 
between the action of the two forms of eye-lens may be illustrated as 
follows : — Form a piece of paper into a shape resembling a speaking- 
trumpet ; now, if the eye be placed at the smaller opening, a field of view 
resembling the field given by a convex eye-glass will be obtained ; reversing 
