Sir D. Brewster on a new Species of Coloured Fringes. 19 



no means of obtaining, nor can 1 state the ultimate deflections; 

 they were not considered of importance, and consequently not 

 registered. With respect to the time, each experiment occu- 

 pied fifteen or twenty minutes. 



VI. On a new Species of Coloured Fringes, produced by Re- 

 flection between the Lenses of Achromatic or Compound Ob- 

 ject-Glasses. By Sir David Brewster, K.H. LL.D 

 F.E.S. V.P.R.S. Ed,* 



[With a Plate.] 

 TN a paper which I communicated to this Society in 1815, 

 -■- and which was published in the seventh volume of their 

 Transactions, I described a new species of coloured fringes, 

 produced l)etween two plates of parallel glass. From a con- 

 sideration of the theory of this class of pha;nomena, it was ob- 

 vious that analogous, though much more complicated, systems 

 of lings should be produced between plates with curved sur- 

 faces, but it was not till 1822 that I succeeded in detectino- 

 them; and so completely are these rings concealed by the 

 superposition of similarly situated images, that, in consequence 

 of having forgotten my method of observation, I have expe- 

 rienced the greatest difficulty in rediscovering them. 



My earliest experiments were performed with a double 

 achromatic object-glass, made by Berge, having a diameter of 

 2y'„ inches, and 30 inches in focal length. The inner surfaces 

 of the crown and flint glass lenses were ground to different 

 radii, as shown in the section of it at AB, CD, Plate I. Fif. 5; 

 and the outer surface of the flint-glass lens was concave, so that 

 there was left between the lenses a meniscus of air A 2 B 3 A. 



In order to observe the system of rings as nearly as possible 

 at a perpendicular incidence, I placed the smallest flame I 

 could procure at S, about four or five inches distant from the 

 object-glass AD, and interposing a small screen G between 

 the flame and the eye at E, I held the eye as close to S as 

 possible, and varied the distance of the object-glass till the 

 inverted greenish-coloured flamef reflected interiorly from the 

 concave surface A 1 B seemed to cover the whole area of the 

 object-glass. When this is accomplished, the rings may, by 

 a slight change in the position of the object-glass, or by screen- 

 ing the image formed by one reflection from A 1 B, be di- 

 stinctly seen over the expanded but enfeebled image formed 

 by a second reflection from the same surface. 



* From the Transactions of the Royal Society of Edinburgh, vol, xii. 

 t 'I'his flame has a greenish colour, in consequence of the rays which form 

 It havnig passed through twice the thickness of the crown "lass lens A B 



D2 



