seen in Achromatic Object-Glasses. ^' '■ 21 



the increase in the reflected light, at increased obliquities of 

 incidence. 



In some object-glasses the rings are exceedingly numerous 

 and close, whether seen as in Fig. 1, or as in Fig. 3; and 

 when this is the case, the black rings ?«^?i, and the centres j:,^, 

 are near the circumference. In other object-glasses, particu- 

 larly in a large one of Tulley's, in the Calton Hill Observa- 

 tory, the rings are very few in number, and the dark fringes 

 mi n, and the centres x^ x, are advanced considerably from the 

 circumference towards the centre of the lens. In this case the 

 rings are more easily seen, and they undergo very beautiful 

 modifications in passing from a perpendicular to an oblique 

 incidence. 



There can be little doubt that this variation in the size and 

 number of the rings depends on the thickness of the meniscus 

 of air between the lenses ; but in order to put this to the test 

 of experiment I separated the two lenses AB, CD, Fig. 5, 

 and I found the rings to increase in number, and diminish in 

 breadth, in proportion to the distance of the two lenses. Hence 

 it follows, that, in all those object-glasses where the inner sur- 

 faces are coincident, or are cemented by mastic or other var- 

 nishes, no rings will be produced, — and that the number of 

 the rings furnish us with a measure of the difference of cur- 

 vature of the inner surfaces of the combined lenses. 



In some of the oblique systems of rings which I have ob- 

 served, the outer fringe n of one of the central systems ap- 

 proached so near the outer fringe m of one of the external 

 systems, that the space between them was stra'w-ycllo'w, in 

 place of white ; and in one case, the four bounding fringes 

 united, and formed a black cross, as shown in Fig. 4. 



In a large double object-glass, made by Gilbert, 3"8 inches 

 in diameter, and in a similar one by Dollond, 2*75 inches in 

 diameter, the rings could only be seen by looking through 

 the convex side A 1 B, Fig. 5. In the first of these lenses 

 there were onlj' two fringes in the near central system'^of rings, 

 so that the inner surfaces must have been nearl}' coincident. 



If we separate the lenses a little at A, Fig. 1. and Fig. 5, 

 the system of rings approaches the edge B, and become more 

 numerous and more close to each other. The other systems 

 close, and become concentric to them, and the whole become 

 an elliptical system. 



When the lenses are separated a little at B, Fig. 1. and 

 Fig. 5, the system enlarges, and the rings grow more nume- 

 rous, the other systems becoming concentric with them, and 

 forming a close system. 



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



