The Barlow Lens

Abstract

I HAVE found the addition of a double concave lens to my telescope and microscope of so much service that I am anxious to call the attention of your readers to this simple application for increasing and improving the working power both of telescopes and microscopes. The application consists in the introduction of a biconcave lens in the adapter, which holds the eye-piece of the telescope, at a distance of two or three inches from the field-lens; as the focal length of the instrument is thereby increased, it is necessary to adjust the distance of the lens from the eye-piece according to the length of the adapter, so that the latter still admits of being drawn out sufficiently for focussing. A friend procured me several lenses of different powers at the ridiculous price of a shilling a-piece from an optician and spectacle-maker at Brighton, which answer admirably.