208 SEC. 7. LIGHT. 



phragm, by closing which upon the glass, the " fiducial " yellow ray trans- 

 mitted by blue cobalt glass will be observed with the accompanying eyepiece 

 of a small pocket spectroscope placed with the plate. Through the narrow 

 slit left between them the selective absorption of the glass can easily be 

 distinguished if white light is examined through it, and spectroscopically 

 analysed by means of the dispersion of the prisms. 



825a. Fluorescent Eyepiece, by M. Soret, for adaptation 

 to the Spectroscope. 



Geneva Association for the Construction of Scientific In- 

 struments. 



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 focus of the objective of the spectroscope. The ultra-purple 

 spectrum projected upon this plate becomes visible, and is observed through 

 an eyepiece which is movable upon the axis of the observing telescope. 

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

 Poggeudorffs Annalen, 1274. Jubelband, p. 407; Archives des Sciences 

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



82 5a. Combination of Three Prisms of different dis- 

 persive power, giving by one combination deviation without dis- 

 persion, and by another dispersion without deviation. Formerly 

 the property of Dr. Priestley. Mrs. Parkes. 



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

 researches in Spectrum Analysis. Robert Sabine. 



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. 



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 ^eyepiece, 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 wire of which is horizontal and moves 

 in a vertical plane. 



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

 connected with the coatings of a Leyden 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 



