78 REPORT — 1853. 



excluding all extraneous light from the eye and gazing at the spectrum fixedly 

 for some time, a trace of the green ray, nearly black, is rendered evident. 



(118) 85. Neither of these three glasses show the slightest transparency 

 to the chemical rays. Not only is there no effect produced by the action of 

 the most concentrated spectrum, but the most sensitive calotype paper may 

 be exposed for many hours to an intense sunshine under them, without show- 

 ing the least signs of chemical change. 



(119). To the photographic artist this fact is of considerable importance, 

 since it gives him the means of excluding absolutely all the chemically active 

 rays, still allowing a sufficient quantity of light to pass to enable him to work 

 without any inconvenience. Yellow glasses have frequently been employed ; 

 — the results shown, however, in this Report prove thenecessity of examining 

 with great care the yellow glass which is used, as it is not the question of in- 

 tensity of colour, but of the physical character of the colouring agent, that 

 regulates the transparency or opacity of the glass to the chemical rays. 



Quinine Solution. 



(120). From the interest attached to the peculiar property of the solution of 

 sulphate of quinine in water by means of dilute sulphuric acid, to bring into 

 view a set of rays bej'ond the violet, of a beautiful celestial blue colour, cor- 

 responding with those produced by the canary-yellow glass, I became anxious 

 to examine the influence of it on the chemical rays. This became the more 

 important, since it had been asserted that the ordinarily dark chemical rays 

 had been rendered visible, and this brought forward as an additional proof 

 of the identity of the rays producing luminous and chemical phsenomena. 



(121). The solution employed was that recommended by Mr. Stokes, as the 

 best for observing the peculiar phaenomena o^ fluorescence, as it has been 

 named, consisting of one part of the sulphate of quinine to 200 parts of water. 

 For the purpose of ascertaining if any greater degree of absorption was pro- 

 duced by using a more concentrated solution than this, experiments were 

 made with such as contained as much as six times this quantity of quinine ; 

 but unless this is distinctly stated, it will be solutions of the first-named 

 strength which were employed. 



(122). A plate-glass trough was used, and being first filled with distilled 

 water, the length and general character of the prismatic spectrum were care- 

 fully observed and determined. The trough was then filled with the before- 

 mentioned solution of sulphate of quinine, the result of which was sufficiently 

 remarkable. The ordinary rays of the Newtonian spectrum passed the solu- 

 tion freely, and formed a well-defined image upon a screen placed to receive 

 them. According to the strength of the solution employed, so more or less 

 of the violet ray was cut off. The absorptive action on the other rays was 

 quite inappreciable. From the mean violet ray diminishing, however, towards 

 the end of the ordinary spectrum, the fluorescent rays penetrate the solution 

 1 inch in thickness, forming a stream of a beautiful celestial blue passing 

 across the fluid ; beyond this, over a space often nearly equal to the length 

 of the ordinary spectrum, the new rays continue in view, but in no case 

 penetrating the fluid. Mr. Stokes's observations may be quoted in confirma- 

 tion of these conditions : 



" In the case of a solution of sulphate of quinine of the strength of one 

 part of the disulphate to 200 parts of acidulated water, it has been already 

 stated that a portion of the rays which are capable of producing dispersed 

 light passed across a thickness of 3 inches. On forming a pure spectrum, 

 the fixed line H was traced about an inch into the fluid. On passing from 

 H towards G, the distance that the incident rays penetrated into the fluid 

 increased with great rapidity, while on passing in the contrary direction it 

 diminished no less rapidly, so that from a point situated at no great distance 



