Dr. Herschei/s Experiments , &c. 1B1 
The great beauty of the coloured rings, and the pleasing 
appearances arising from the different degrees of pressure of 
the two surfaces of the glasses against each other when they 
are formed, and especially the importance of the subject, have 
often excited my desire of enquiring farther into the cause of 
such interesting phenomena ; and with a view to examine them 
properly I obtained, in the year 1792, the two object-glasses 
of Huygens, in the possession of the Royal Society, one of 122, 
the other of 170 feet focal length, and began a series of expe- 
riments with them, which, though many times interrupted by 
astronomical pursuits, has often been taken up again, and has 
lately been carried to a very considerable extent. The conclu- 
sions that may be drawn from them, 'though they may not per- 
fectly account for all the phoenomena of the rings, are yet suffi- 
ciently well supported, and of such a nature as to point out 
several modifications of light that have been totally overlooked, 
and others that have never been properly discriminated. It will, 
therefore, be the aim of this paper to arrange and distinguish 
the various modifications of light in a clear and perspicuous 
order, and afterwards to give my sentiments upon the cause 
of the formation of the concentric rings. The avowed intricacy 
of the subject,* however, requires, in the first place, a minute 
detail of experiments, and afterwards a very gradual develope- 
ment of the consequences to be deduced from them. 
As the word modification will frequently be used, it may 
not be amiss to say, that when applied to light, it is intended 
to stand for a general expression of all the changes that are 
made in its colours, direction, or motion : thus, by the modifi- 
* Newton’s Optics, 4th ed. p. 288 ; end of Obs. 12. 
MDCCCVII. B b 
