seen in Achromatic Object-Glasses. 195 



In a triple object-glass, which gave a system of rings similar 

 to that in Fig. 3, I observed them to be crossed with another 

 system of minute fringes parallel to one another, and to the line 

 joining the centres on and an. The object-glass which exhibited 

 this curious effect is not now within my reach, so that I am un- 

 able to give any farther account of this new system. 



In order to determine the surfaces of the double object-glass, 

 AD, Fig. 5, which are essential to the production of the rings, 

 I covered the convex surface A1B with oil of nearly the same re- 

 fractive power as glass, and the rings wholly disappeared. Having 

 removed the oil, I filled with the same fluid the space or hollow 

 meniscus between the lenses, when the rings again disappeared. 

 The lenses being again cleaned, I removed CD, and could no 

 longer observe any fringes. Hence, it follows, that the action of 

 the two surfaces of the convex lens, and the inner surface of the 

 concave one, are necessary to the production of the fringes. 



From these facts, it will appear, that the coloured rings arise 

 from the interference of two pencils of light, one of which has 

 suffered three reflexions within the convex lens AB, and has 

 passed four times through its thickness, with another pencil 

 which has suffered two reflexions within the convex lens, and one 

 reflexion from the inner surface of the concave lens, and has 

 passed four times through the thickness of the convex lens, and 

 twice through the thickness of the meniscus of air. 



When the light is incident perpendicularly on the centre of 

 the lens, the interval of retardation, or the difference between 

 the lengths of the paths of the two rays, is equal to twice the 

 greatest thickness of the meniscus of air. Hence, if this thick- 

 ness is very small, the tints corresponding to it will be distinctly 

 observed ; but if the thickness is considerable, as it often is, the 

 tints will belong to such high orders, that they will only be seen 

 when a small flame of homogeneous light is used. 



As the incident ray advances from the centre towards the cir- 

 cumference, the meniscus of air diminishes in thickness, and al- 



