388 Professor PlayfaiPs Description 
though the length of the spectrum is the same in the two prisms 
of tlie same substance, and with different refracting angles, yet 
the coloured spaces are hot proportional ; and that there is a 
tertiary spectrum produced in all such opposite refractions. 
In the eulogium which Professor Amici has pronounced on 
the influence of experiment in physical researches, I heartily 
concur ; but though an experiment directed to Another object 
conducted him to the property in question, yet with me the case 
was quite the reverse ; for I had deduced it solely from theory, 
as I have stated in my treatise, before I had made an experi- 
ment on the subject. 
The coincidence between Professor Amici’s researches and 
mine, must no doubt have been purely accidental, and it is not 
to be wondered at, that experiments recorded in the English 
language should be unknown in Italy a long time after they were 
made. I confess, however, it does surprise me, that in the very 
memoir of which we are speaking, Professor Amici has quot- 
ed, by page, my Treatise on New Philosophical Instruments, 
and that this quotation relates to the same subject, and is taken 
from the same chapter, which contains all my investigations re- 
specting the production of colourless refraction by two prisms 
of the same substance. 
Although I have felt it my duty to make this statement, I 
trust it will not be understood that I impute any blame to 
such a respectable and eminent philosopher as Professor Amici, 
and I have no doubt that he will be able to give a satisfactory 
explanation of any of those circumstances, which may appear to 
himself to require it. 
Art. 'KNll.^Description the Slide of Alpnach. By the 
late John Playfair, Esq., Professor of Natural Philosophy 
in the University of Edinburgh, & Sec. B. S. E. With 
Notes and Observations. 
« On the south side of Pilatus, a considerable mountain near 
Lucerne, are great forests of spruce fir, consisting of the fixnest 
• From his Wot'Ics, in four volumes octavo, just published, vol. i., A^ypendix 
No. 2. p. Ixxxix. See this Journal^ Voh I. p. 193. and Vol. II. p. 110. 
