The details of
these later methods are now well known, and have been extensively
practised. Many amateurs have thus been able to make telescopes by
following the instructions so clearly laid down by Lord Rosse and the
other authorities. Indeed, it would seem that any one who has a
little mechanical skill and a good deal of patience ought now to
experience no great difficulty in constructing a telescope quite as
powerful as that which first brought Herschel into fame. I should,
however, mention that in these modern days the material generally
used for the mirror is of a more tractable description than the
metallic substance which was employed by Herschel and by Lord Rosse.
A reflecting telescope of the present day would not be fitted with a
mirror composed of that alloy known as speculum metal, whose
composition I have already mentioned. It has been found more
advantageous to employ a glass mirror carefully figured and polished,
just as a metallic mirror would have been, and then to impart to the
polished glass surface a fine coating of silver laid down by a
chemical process. The silver-on-glass mirrors are so much lighter
and so much easier to construct that the more old-fashioned metallic
mirrors may be said to have fallen into almost total disuse.
Pages:
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233