for altering the Lineal Proportions of Objects. 3ST 
not seem to have entered his mind to use it for the purposes to 
which I applied it. 
* With regard to the experiments and optical principles upon 
which the instrument is founded, it is necessary to state, that 
Professor Amici has published as new, in 1821, a series of ex- 
periments almost exactly the same as those which were publish- 
ed by me in 1813, in my Treatise on New Philosophical In- 
struments. It has hitherto been believed, says this ingenious 
author, by natural philosophers, that the dispersion of colours is 
constant for the same refracting medium, or that a given re- 
fraction produced by the same substance is accompanied by a 
given dispersion ; but I have found that the dispersion produced 
by more than one refraction is not by any means constant, but 
varies according to the various inclinations of the incident ray/’ 
In concluding his memoir, he goes on to observe, ‘‘ That the 
ordinary theory of prismatic colours may easily shew us, that 
achromatic refraction does not necessarily require more than one 
refracting substance ; and that though this theory has been 
deeply studied by so many distinguished opticians and mathema- 
ticians, from the time of Newton to the present day, yet the pro- 
perty here described not only remained unlcnown^ but would 
have been reckoned impossible, if I had not discovered it in a 
series of experiments, which I made for a different purpose. 
We thus obtain an example, to add to so many others, that in 
physical science, experiment is very often., and perhaps most 
commonly more successful than theory, in developing all the cir- 
cumstances which accompany a given phenomenon.” 
Now, in the work already quoted, I have demonstrated by 
direct experiment, as well as by theory, that the refraction is 
not constant for the same refracting medium; — that the dis- 
persion varies with the inclination of the incident ray ; — and that 
r fraction without colour may he produced by two prisms of the 
same substance. I have described, in short, in the fullest man- 
ner, that property, which, according to Professor Amici, has 
been hidden from philosophers from the days of Newton to 
the time of his discovering it. By examining Chap. I. of the 
fifth Book of the work already quoted, it will be seen, that I 
ha;d pushed the inquiry still farther than the Italian philoso- 
pher. I have shewn by experiment, as well as by theory, that 
