II. SELECTORS. 179 



narrow slit left between them the selective absorption of the glass, when ordi- 

 nary white light is examined and spectroscopically analysed by means of the 

 dispersion of the prisms. 



825a. Fluorescent Eye-piece, by M. Soret, for adaptation 

 to the Spectroscope. 



Geneva Association for Constructing Scientific Instruments. 



It consists of a plate made of a fluorescent and transparent substance 

 (uranium glass or different liquids contained between two glass plates), which 

 is placed in the glass of the spectroscope at the focus of the objective. The 

 ultra-purple spectrum projected upon this plate becomes visible, and it is 

 observed through an eye-piece which is inclinable upon the axis of the glass. 

 A very intense light is necessary. (For description of this apparatus, see 

 Poggeudorff's Annalen, 1274. Jubelband, p. 407; Archives des Sciences 

 physiques et naturelles, 1874, vol. 41, p. 338 ; and 1875, vol. 54, p. 255.) 



82 5b. Apparatus used by Sir C. Wheatstone in his early 

 researches in Spectrum Analysis. R. Saline. 



82 5c. Arrangement of Apparatus for Experiments on 

 the Assay of Gold Alloys, by means of the Spectroscope, 



in the manner suggested by Mr. Lockyer. 



W. Chandler Roberts, F.R.S., Chemist of the Mint. 



The apparatus consists of: 



(1.) An induction coil, capable of giving a 10-inch spark in air, which is 

 provided with a Foucault contact breaker in order that the spark may be 

 perfectly continuous. 



(2.) A frame on which the portions of metal under examination can be so 

 arranged as to be easily brought in succession under a fixed pole of alumi- 

 nium. Accompanying this frame is a fixed microscope provided with cross 

 wires in the eye-piece, and the table bearing the assay pieces can be adjusted 

 by a micrometer screw, so that the image of the apex of each assay piece can 

 be brought to coincide with the cross wires, thus ensuring that the striking 

 distance remains constant. 



(3.) A lens to throw an image of the spark on the slit of 



(4.) A large spectroscope In which the spectra of the alloys are examined. 

 It is provided with a micrometer, the wires of which are horizontal and move 

 in a vertical plane. 



When a spark from the induction coil (the two terminals of which are also 

 connected with the coatings of a Ley den jar) passes between the aluminium 

 pole and one of the alloys, an atmosphere of the vapours of gold and copper 

 is formed round the lower pole which does not extend to the upper pole, and 

 therefore in the spectrum observed the lines due to these metals will not cross 

 the field of view. Mr. Lockyer observed, that, when all other conditions 

 remain the same, if the composition of the alloy be slightly altered, the relative 

 heights and intensities of the lines of the two metals vary. For these com- 

 parisons the gold line having a wave length of 5,230 tenth-metres, and the 

 copper line 5,217, are the most convenient. If a series of known alloys vary- 

 ing slightly in composition is examined, a curve maybe constructed, the ordi- 

 nates of which represent the ordinary assays, and the abscissae the micrometer 

 readings for the points at which the above two lines are equally bright, and 

 then, theoretically, if an unknown alloy of about the same composition be 

 examined, this curve enables us to determine its exact composition when the 

 micrometer reading is known. 



M 2 



