Page:The wonders of optics (1869).djvu/135

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word lens is derived from the Latin name of the seed of the Ervum lens, or ordinary lentil. When eating this wholesome vegetable, almost every one has noticed that its shape is exactly that of a double convex lens, as represented in the following figure:—



Perhaps it would be more correct if we were to say that a double convex lens is like a lentil, rather than turn the comparison the other way, seeing that this little seed has given its name not only to the particular-shaped glass depicted above, but also to some five others more or less analogous to it.

In fig. 27 we have the different forms of lenses