PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. o4l 
quartz vessel was partly filled with water, the addition of a very small quantity of 
nitrate of iron was sufficient to cause the absorption of the active rays. The solution 
was so weak as to be almost colourless when viewed through the thickness through 
which the rays would have to pass. A solution of perchloride of iron had a similar 
effect. These fluids I had specially examined by sunlight, and had not found in them 
the least trace of internal dispersion. When a fluid exhibits internal dispersion, it is 
almost always very opake with regard to rays of high refrangibility, as is shown, 
without any special experiment, in the course of the observations by which the internal 
dispersion is exhibited ; but it by no means follows conversely, that when a fluid is 
very opake with regard to these rays, though nearly transparent with regard to the 
visible rays, it exhibits the phenomenon of internal dispersion. 
204. I have little doubt that the solar spectrum would be prolonged, though to 
what extent I am unable to say, by using a complete optical train in every member 
of which glass was replaced by quartz. Such a train would be rather expensive, but 
would not involve any particular difficulty of execution. If solid prisms of quartz 
were used, half of the incident light would be lost, on account of the double refraction 
of the substance, unless the prisms were cut in a particular manner, which however 
would seem likely to involve some difficulties, both in the execution and in the ob- 
servations. But hollow prisms holding fluids might be employed, having the two faces 
across which the light has to pass made of quartz plates. For a reason already men- 
tioned, sulphuret of carbon cannot be employed for filling the prisms, and the disper- 
sive power of water is very low, but there appears to be no objection to the use of a 
solution of some colourless metallic salt. At least saturated solutions of sulphate of 
zinc and of acetate of lead, the only salts I have tried with this view, showed no 
defects of transparency when examined in quartz vessels by means of the flame of a 
spirit-lamp and a solution of sulphate of quinine*. 
Effect of Hydrochloric Acid, 8^c. on Solutions of Quinine. Optical evidences of combi- 
nation in other instances. 
20.5. 8ir John Herschel, in his interesting paper already so often referred to, ob- 
serves that it is only acid solutions of quinine which exhibit the peculiar blue colour, 
and that among different acids the muriatic seems least efficacious (page 145). 
For my own part I have tried solutions of quinine (not disulphate) in dilute sul- 
phuric, phosphoric, nitric, acetic, citric, tartaric, oxalic, and hydrocyanic acids, and 
also in a solution of alum. In all these cases the blue colour of the dispersed light 
was plainly seen by ordinary daylight, especially when the fluid was examined by 
superficial projection. It was not easy to say which solution answered best, but I 
am inclined to think that in which phosphoric acid was used. 
206. But when quinine was dissolved in dilute hydrochloric acid the blue colour 
was not exhibited, not even when the fluid was held in the sunlight, and examined 
by superficial projection, (’ertain theoretical views led me to regard this as an evi- 
* See note H. 
