Ixxii Proceedings of the South African Philosophical Society. 



advancement of those separate sciences. I may perhaps be per- 

 mitted to dwell briefly on this most interesting subject. The fact 

 that light travels with a measurable velocity was first demonstrated 

 by Eoemer in 1675, because he found that the eclipses of Jupiter's 

 Satellites apparently occurred too soon when the earth is near 

 Jupiter and apparently too late when far from Jupiter — a phenomenon 

 that could only be accounted for on the assumption that light travels 

 with a measurable velocity. Here, then, is a notable contribution by 

 the old astronomy to the science of physics. Newton, as every one 

 knows, showed how sunlight, after passing through a slit and a 

 prism, was split up into its component colours. Fraunhofer in 1815 

 proved that this spectrum, or coloured ribbon, viewed with more 

 perfect appliances than Newton employed, is crossed by fine dark 

 lines ; in other words, that certain very definite kinds of refrangibility, 

 or colours of light, are wanting in the solar spectrum. Fraunhofer 

 actually measured the position in the spectrum of 600 of these 

 lines, but their significance remained a mystery until 1859, when the 

 explanation w^as found by Kirchhotf and Bunsen. They showed 

 that substances in a state of vapour absorb rays of the same 

 refrangibility as they themselves, when sufficiently heated, emit — 

 and that the dark lines in the solar spectrum are produced by the 

 absorption of vapours of metals, &c., w^hich exist in the solar 

 atmosphere, many of these metals, &c., being the same as those with 

 which we are familiar, such as iron, calcium, sodium, magnesium, 

 hydrogen, &c. With this discovery the new astronomy sprang into 

 life. Sir William Huggins, the present President of the Eoyal 

 Society, was the first to apply, in a really crucial and scientific 

 manner, this new engine of research to other systems than our own. 

 With infinite labour and ingenuity he designed and had constructed 

 a spectroscope apphcable to analysis of the light of celestial objects. 

 It was requisite that this spectroscope should be mounted on a 

 telescope, so that the comparatively faint light of a star might be 

 collected by the object glass, and be projected at its focus on a slit of 

 one or two thousandth parts of an inch in width, and be retained 

 steadily on that slit, in spite of the diurnal motion of the earth. 

 Farther, it was necessary to provide means by which the infinitely 

 small point of light formed by the star's image should be widened, so 

 that there should be seen in the field of view, not a mere coloured 

 line, but a coloured ribbon of appreciable width. Finally, means 

 had to be contrived for introducing into the slit (just as if it had 

 come from the star), the light given off by terrestrial substances in a 

 state of incandescence, so that the dark lines in the spectrum of the 

 star might be compared with the bright lines of the spectra of 



