[Illustration with caption: GRINDING A LARGE LENS.]
When the polishing is done by machinery, which is the custom in
Europe, with large lenses, the polisher is slid back and forth
over the lens by means of a crank attached to a revolving wheel.
The polisher is at the same time slowly revolving around a pivot
at its centre, which pivot the crank works into, and the glass
below it is slowly turned in an opposite direction. Thus the same
effect is produced as in the other system. Those who practice this
method claim that by thus using machinery the conditions of a
uniform polish for every part of the surface can be more perfectly
fulfilled than by a hand motion. The results, however, do not
support this view. No European optician will claim to do better
than the American firm of Alvan Clark & Sons in producing
uniformly good object-glasses, and this firm always does the work
by hand, moving the glass over the polisher, and not the polisher
over the glass.
Having brought both flint and crown glasses into proper figure by
this process, they are joined together, and tested by observations
either upon a star in the heavens, or some illuminated point at a
little distance on the ground. The reflection of the sun from a
drop of quicksilver, a thermometer bulb, or even a piece of broken
bottle, makes an excellent artificial star.
Pages:
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128