s 3© Dr, Herschei/s Experiments for investigating 
XXXIII. Coloured Rings may be completely formed without the 
Assistance of any thin or thick Plates , either of Glass or of Air. 
The experiment I am now to relate was at first intended 
to be reserved for the second part of this paper, because it 
properly belongs to the subject of the flection of the rays 
of light, which is not at present under consideration ; but as it 
particularly opposes the admission of alternate fits of easy 
reflection and easy transmission of these rays in their passage 
through plates of air or glass, by proving that their assistance 
in the formation of rings is not required, and also throws 
light upon a subject that has at different times been considered 
by some of our most acute experimentalists, I have used it at 
present, though only in one of the various arrangements, in 
which I shall have occasion to recur to it hereafter. 
Sir I. Newton placed a concave glass mirror at double its 
focal length from a chart, and observed that the reflection of 
a beam of light admitted into a dark room, when thrown 
upon this mirror, gave “ four or five concentric irises or 
rings of colours like “ rainbows.”* He accounts for them 
by alternate fits of easy reflection and easy transmission ex- 
erted in their passage through the glass-plate of the concave 
mirror.-f 
The Duke De Chaulnes concluded from his own experi- 
ments of the same phenomena, “ that these coloured rings 
depended upon “ the first surface of the mirror, and that the 
“ second surface, or that which reflects them after they had 
*' passed the first, only served to collect them and throw them 
• Nswton’s Optics, p. 265. f Ibid, p, 277, 
