So with two prisms of the same kind
of glass, but of different angles, two spectra can be produced of
exactly equal length, so that if one is brought over the other, with
the corresponding colors in line, they will appear as one spectrum.
But if one of the prisms is made of crown and the other of flint
glass, then their spectra cannot be arranged so that all their
corresponding colors would be in line, for the proportional distances
of the different colors differ in the two spectra. If two colors of
the spectra are, by suitable arrangement, brought exactly in line,
then the others will be out. The two spectra do not coincide, and the
result, if an achromatic lens be made of these glasses, must be a
remnant of color which cannot be neutralized. This remnant is the
secondary spectrum.
Although this peculiar disharmony in the dispersive powers of the two
glasses, crown and flint, was discovered almost immediately after
achromatism was invented, it was only recently that the first
successful attempts were made to produce different glasses, which,
possessing the other requirements for achromatic objectives, would
produce coincident spectra, or nearer so than the ordinary crown and
flint glass do. It was about twelve years ago, if my memory serves me,
when I learned that a well-known English firm, engaged in the
manufacture of optical glass, had brought out some new glass possessed
of the desired qualities, and a little later I received a circular
describing the glass.
Pages:
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159