In the double-concave lens is negative,
In the double-convex is negative,
In the plano-concave either is infinite, or is infinite, and negative; therefore putting for the single radius
In the plane-convex,
When in the double-concave, or double-convex lens the radii are
equal,
[1]
88.It appears from all this, that the place of the principal focus is the same, whichever side of a lens is turned towards the incident light, and that
The concavo-convex[2] |
|
make parallel rays diverge.
|
the double-concave
|
and the plano-concave
|
The meniscus |
|
make parallel rays converge.
|
the double-convex
|
and the plano-convex
|
- ↑ If which is nearly the case in glass, or the principal focal length is equal to the radius of sphericity.
- ↑ See Fig. 87, for the relation between these different kinds of lens. Those placed together are equivalent.