92 Dr. H. Morton on the Fluorescent Relations of certain 



same, except that the spectroscope was turned round into the posi- 

 tion indicated in fig. 3, and the stand for specimens was replaced 



Fig. 3. 



by a plane table D, on which the substance to be studied was 

 supported either in a cell of glass or in a bottle. White glass 

 bottles of about one-ounce capacity formed cylindrical lenses of 

 a very convenient character for this purpose. 



To examine the relative exciting-power of the various colours 

 of the spectrum, an arrangement, nearly identical with that 

 described by Professor Stokes as his "Third Method"*, was 

 employed. 



Fig. 4. 



In this case the light thrown in by the porte-lumiere A passed 

 through a narrow adjustable slit at B, then encountered a prism 

 at C, and after this a lens of about 12-inches focus at D, by 

 which an image of the spectrum was thrown on a screen or tank 

 at E. A spectroscope placed behind this screen was employed, 

 in a manner presently to be described, to measure the refran- 

 gibility of certain rays. 



Thus, suppose a pure spectrum showing the Eraunhofer lines 

 clearly to be thrown on a screen coated with some fluorescent 

 body placed at E, and that a point of maximum or minimum 

 fluorescence were observed, and we wished to determine accu- 

 rately the refrangibility of the rays to which this effect was due. 



A fine needle was made to pierce the paper at this point ; and 

 the light thus allowed to pass through the screen was then mea- 

 sured by the spectroscope. Of course the hole, once made, could, 

 by movement of the screen, be brought into coincidence with 

 any point in the spectrum. 



* Philosophical Transactions, 1852, Part II. p 4/0. 



