HOW TO WOKE WITH THE TELESCOPE. 
467 
in an astronomical telescope (unless it is to be used for comet- 
seeking) no power so low as not to call into play the full 
light-gathering power of the telescope should be used. 
That the moon may be observed to advantage with a 
small telescope, is clearly shown by the excellent map con- 
structed by Beer and Madler with a telescope (by Fraunhofer) 
only 4J feet in focal length. Much yet remains to be learnt 
of the moon's face. The observer who would wish to be ser- 
viceable in this way should select a small portion of the 
moon's surface for diligent examination. After carefully 
studying this part in a good map, he should scrutinize it in 
the original, endeavouring to detect, if possible, details not 
exhibited in his map. He should note the changes of appear- 
ance presented, not only with the changing lunar phases, but 
(especially in districts near the moon's limb) with the varia- 
tions in the epochs of maximum libration. He should be 
prepared to attribute apparent changes of appearance to 
such variations of position ; but any suspicion he may have, 
after careful scrutiny, of actual change taking place upon the 
lunar surface, should be noted for confirmation or disproof by 
other observers. 
In observing the sun the eye must be protected by dark 
green or neutral-tint glasses having parallel surfaces, and 
it is convenient to have an eye-piece arranged with a rotating 
wheel by which darkening glasses of different power may be 
brought into use as the varying illumination may require. 
For observation of minute portions of the sun the Hawes's 
eye-piece should be used : in this a metallic screen placed in 
the focus keeps away all light but such as passes through a 
minute hole in the diaphragm. A very convenient way of 
diminishing the light is to use a glass prism, light being par- 
tially reflected from one of the exterior surfaces, and the 
refracted portion thrown out at another. A pleasant method 
of observing the sun is to project the sun's image on a screen 
placed (in a darkened room) at about 6 or 7 feet from the end 
of the telescope ; a totally reflecting diagonal eye-piece being 
used, otherwise the telescope would have to be erected 
at an inconvenient height. Very beautiful and interesting 
views (on a smaller scale, however) may be obtained by using 
such a pyramidal box as is depicted in Plate XXII., fig. d. 
the instrument, it will be found that, by moving the eye over the open 
space at the large end of the tube (held in a fixed position), every object 
included in the former field can be seen in turn. Of this nature is the 
field, if field it can be called, given by a concave eye-glass, used as suggested 
by Sir W. Herschel. 
