194 HERSCHEL ON COLOURED RINGS* 



the secondary set of rings in view, I found their shape and 

 colour always completely well formed. 



This experiment was also repeated with a small plain glass 

 instead of the metalline minor put under the large plate. In 

 this manner it still gave the same result, with no other differ- 

 ence but that only six rings could be distinctly seen in the 

 secondary set, on account of the inferior reflection of the sub- 

 jacent glass. 



XXXIII. Coloured Rings may be completely formed with~ 

 out tlie Assistance of any thin or thick Plates^ either of Glass 

 or of Air. 



The rings may The experiment I am now to relate was at first intended 



be formed to be reserved for the second part of this paper, because it 



•without thick 5 n . 



or thin plates properly belongs to the subject of the flection of the rays 



of glass or air. f light, which is not at preseut 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. 

 Experiment of Sir I. Newton placed a concave glass mirror at double its 

 Sir I.Newton, f 0CB \ i en gth 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. 

 of the duke of ^ ie Duke de Chaulnes concluded from his own experi- 

 Chaulnes, ments of the same phenomena, " that these coloured rings 

 depeuded upon " the first surface of the mirror, and that the 

 u second surface, or that which reflects them after they had 

 , " passed the first, only served to collect them and throw them 



* Newton's Optics, p. 265. f Ibid, p. 277. 



a upon 



