VISUAL, ORATING AND GLASS-LENS, SOLAR SPECTRUM. 521 



any gaseous matter, and if so, of what kind and in what quantity — accompany 

 the ultra triturated dust of material shot up from earth's furnaces far beneath the 

 sea ?. In the red prominence explosions of the sun, which are far more like the 

 gigantic Krakatao upshoot, than are any of the cannon-ball experiments at Wool- 

 wich, gaseous matter is conspicuous enough. But in the case of any earthly 

 volcanoe, such a conjectured material has never yet been proved for its superior 

 manifestations. For though many persons may — like the elder Pliny when he 

 ventured too near Mount Vesuvius at its classic eruption in a.d. 79, — may, I 

 repeat, have perished from noxious gases exuding out of the lower flanks of 

 the mountain, — those cases do not seem to have excited much curiosity as to 

 whether such volcanic gases are chemically different from those already known 

 to exist in the atmosphere, and are ever ejected in such quantity and with such 

 force as to form a notable part of the explosion in the higher regions of the air. 



The spectroscope however is, with the assistance of transmitted light, an 

 infallible test as to whether any particular medium in the upper air, dense 

 enough to colour the rays of the sun when shining through it, — is formed of 

 solid insoluble particles only, or is composed of a true but strange gas, unnatural 

 to our atmosphere. For in the former case the solar spectrum will be merely, 

 but continuously dulled from one end to the other, — while in the latter some 

 remarkable localised spaces or transverse lines of absorption, in addition to 

 all those already known as Fraunhofer lines, whether of solar or terrestrial 

 origination, may be expected to be met with. 



Hence a solar spectroscoping in 1884, besides its own proper uses for 

 cosmical knowledge, might be expected to have some further special interest 

 connected with our own earth, i.e., if conducted with sufficient dispersive and 

 magnifying power ; and unprecedented power of the former kind has been 

 lately given to many scientists by the magnificent diffraction gratings prepared 

 by Professor Eowland of Johns Hopkins University, Baltimore, on his novel and 

 admirable ruling engine. 



I was among the happy number to receive one of those gratings, with a 

 surface ruled at the rate of 14,438 lines to the inch, over an area of 3*5 x 5 

 inches ; but had intended to confine its use to vacuum tube spectra, — until I 

 learned that the mysterious attractions of the invisible, over the visible, 

 especially when brought out by the hasty, labour-saving method of photography, 

 — were leading most of the other donated observers to neglect the visual portion 

 of the solar spectrum, in spite of its beauty, its central character, and the 

 wonderful organs by which the Creator has enabled man to enjoy it. 



Without giving up therefore my own eventual hopes and intentions on 

 artificial spectra, or interfering in the slightest degree with the recondite 

 proceedings of the greater physicists on the invisible extensions at either end 

 of the solar visible spectrum, — I proceeded through the autumn, winter, and 



