ON SPECTROSCOP Y. 1 5 



now make use of that of the electric arc, the [result is that this blue 

 line becomes extremely developed, and two additional lines in the 

 violet make their appearance, corresponding in position with the 

 two H lines in the solar spectrum. Now, in the sun, these two H lines 

 are the thickest in the whole spectrum, and the blue calcium line before 

 named is comparatively thin. The conditions in the spectrum of calcium 

 in the arc are just the reverse of this, the blue line being very much 

 thicker than the two violet lines. These facts suggested to Mr. 

 Lockyer that between the temperature here employed and that of the 

 sun, there should be a difference which affects the spectrum of 

 calcium in a corresponding manner, as the difference in temperatures 

 at our command affects the spectrum of the chloride of calcium. To 

 verify this, Mr. Lockyer made direct experiments upon calcium at 

 different temperatures, using for this purpose a large induction coil 

 and interpolating Leyden jars of different sizes, and by these means he 

 obtained at the lowest temperature the blue line without any trace of 

 the two violet lines, whilst at the highest the two violet were present 

 and the blue entirely absent. From these results, Mr. Lockyer argues 

 that here we may have a dissociation of calcium itself, seeing that a 

 parallel exists between them and the changes taking place during the 

 dissociation ot the chlo/ide. In conclusion, Mr. Lockyer justly 

 observed that this work was one which would have taken a single 

 individual very many years to accompli sh,but as portions had been 

 taken up by other workers at Owens College, Manchester, and at 

 Potsdam, a completion of this important work will be brought about in, 

 much less time. 



Professor SORET (of Geneva) recalled that he gave, two years ago, 

 the description of a spectroscope with a fluorescent ocular glass 

 (Bibliothcque Universelle Archives, Sc. phys. et nat. 1874 t. xlix., 

 p. 338.) The essential disposition consists in placing at the focus of 

 the telescope of an ordinary spectroscope, a thin plate of a transparent 

 and fluorescent substance, such as glass of uranium or a layer of a 

 solution of esculine between two lamels of glass. The ultra-violet spec- 

 trum is formed upon this plate, as in the celebrated experiments of 

 Professor Stokes, and it can be perceived with an ordinary positive 

 ocular glass, inclined towards the axis of the telescope. 



This ocular glass was first applied to a spectroscope of which the 



