Instrumental Aids to Observation.
The powers of the eye, acute as they are, have narrow limits; inestimable therefore is the value of the microscope, the telescope and the camera which bring to view uncounted images otherwise unseen. Let us remark how in the early days of instrumental aids a great observer just missed noting a phenomenon of utmost importance,—the black lines of the solar spectrum, upon which Fraunhofer, an optician of Munich, based his spectroscope. In sending a solar beam through a lens and a prism Sir Isaac Newton admitted the rays through an oblong slit at times as narrow as one twentieth of an inch. He saw the familiar colors, from red to violet, and nothing more. Even with a crown lens, such as he probably used, four lines distinctly appear; that is, they appear to-day, to an observer who is looking for them. In 1802 these lines were observed, as far as we know, for the first time on record, by Dr. Wollaston, who drew six of them in a diagram accompanying a paper in the Philosophical Transactions. Four of these lines he regarded as boundaries of the colors of the spectrum; of the other two lines he attempted no explanation. He used prisms of various materials but found no alteration in the lines while he studied a sunbeam. When he employed candles or an electric light he found the appearances different, why, he could not undertake to explain. In 1814, Fraunhofer observed these lines in detail, mapped them, and proved that they identified elements long known to chemists. As he built his spectroscope he gave the chemist, the physicist and the astronomer an instrument of research worthy a place beside either the microscope or the telescope.
Dr. Wollaston, in 1802, as we have seen stood upon the threshold of spectroscopy without knowing it. During the same year he performed an experiment which took him into the field of photography without his recognizing the possibilities of that wonderful art. He took paper which had been dipped in muriate of silver and caught on its surface impressions of the ultra-violet light in a solar spectrum. These rays, as rings, were reflected from a thin plate of air, as in the case of the colors of thin plates, at distances corresponding to their proper places in the spectrum. Thus was established the close analogy between rays visible and invisible, and by a method destined to give mankind a universal limner in light of all kinds, and in much radiance which is not luminous at all.