87. Now in the first place it will be immediately seen that this expression gives the principal focal distance, which we will call , by leaving out the last term, which is equivalent to making , or infinite: we have thus
It appears from the former of these that is positive or negative according as is so: let us examine what sign this is affected with in different cases.
In the concavo-convex lens placed as in Fig. 86, and is positive.
When this lens is turned the contrary way, , but they are both negative, we have then
and is positive as before.
In the meniscus, either both being positive, and then
or , and both are negative: so that
↑It is often found convenient to put some symbol such as for which gives , or . When the radii are equal in a double concave or convex lens .