Abstract
The importance, as an instrument of research, which the spectroscope has reached within a few years, renders any improvement therein a matter of general scientific interest. Hitherto it has been under a disadvantage, which, though slight in amount in those cases in which the dispersive power of the instrument is moderate, becomes a rather serious annoyance to the observer when a number of prisms are used in serial combination, and the curvature of the spectral lines is proportionally increased, and only to be restrained appearance by using a narrow breadth of the spectrum. I have lately thought of a very simple and practical remedy (which may indeed have occurred to others, but which I have not seen mentioned), whereby those lines are rendered palpably straight in a very large field; but previous to describing it, it is desirable to refer to a statement appearing in the ‘ Astronomical Notices ’ for last month (March), viz. that the spectral lines can be rendered perfectly straight simply by returning them (after their first passage through a series of prisms arranged for minimum deviation) by a direct reflection from a plane mirror ; and, further, that this has been accomplished in a spectroscope in construction for the Royal Observatory.
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