Ss. 
ee 
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REFRANGIBILITY of LIGHT. 25 
ed towards a diftant object, the rays will enter it, as to fenfe, 
perpendicularly, and will therefore fuffer no refraction. If the 
convex furface of this lens be brought in contact, with a fluid 
. of lefs mean refrative denfity than the glafs, but exceeding it 
in difperfive power, in that degree which occafions an equal re- 
fraGtion of all the’ rays, all thefe rays will then’ be converged to 
the fame point, which are incident at the fame diftance from 
the axis of the lens. The focal diftance of this compound lens 
will be greater or lefs in proportion to its radius of convexity, 
and to the difference of refraction between it and the fluid made 
ufe of. While the fluid is confined on one fide by the plano- 
convex lens, let the lens which is brought in contac with it on 
the oppofite fide, have one of its fides ground convex, and the 
other concave ; the radii of their fphericities being equal to the 
focal diftance at which the rays are made to converge, by the 
refraction which takes place, when light paffes from the plano- 
convex lens into the fluid. It is manifeft that the light will 
now both enter into this compound lens, and emerge from it 
perpendicularly, and will therefore fuffer no refraction, except 
in the confine of the convex fide of the plano-convex and the 
difperfive fluid, where all the rays are equally refrangible. A 
compound lens of this kind, is reprefented in the ninth figure, 
which, after what has been faid, will require no farther expla- 
nation ; excepting only, that inftead.of being fpherical, it is re- 
prefented with that curvature which converges homogeneal rays, 
incident at all diftances from the axis, to the fame point. 
If the required curvature could be given to lenfes with fuffi- 
cient accuracy, this figure feems to réprefent as perfect a con- 
ftruction of the object-glafs of a telefcope as can be defired. 
But there is reafon to think that a fpherical figure may be com- 
municated, not only much eafier, but with greater accuracy than 
a fpheroidal or hyperboloidal, which would be required ; and 
even if this difficulty could be got over, there would ftill re- 
main a fundamental fault in the theory. Before relating the 
Vor. III: D obfervations 
