Mrs. B. The microscope is constructed for this purpose. The single microscope ([fig. 5.]) consists simply of a convex lens, commonly called a magnifying glass; in the focus of which the object is placed, and through which it is viewed: by this means, you are enabled to place your eye very near to the object, for the lens A B, by diminishing the divergence of the rays, before they enter the pupil C, makes them fall parallel on the crystalline humour D, by which they are refracted to a focus on the retina, at R R.
Emily. This is a most admirable invention, and nothing can be more simple; for the lens magnifies the object, merely by allowing us to bring it nearer to the eye.
Mrs. B. Those lenses, therefore, which have the shortest focus will magnify the object most, because they enable us to place it nearest to the eye.
Emily. But a lens, that has the shortest focus, is most bulging or convex; and the protuberance of the lens will prevent the eye from approaching very near to the object.
Mrs. B. This is remedied by making the lens extremely small: it may then be spherical without occupying much space, and thus unite the advantages of a short focus, and of allowing the eye to approach the object.
There is a mode of magnifying objects, without the use of a lens: if you look through a hole, not larger than a small pin, you may place a minute object near to the eye, and it will be distinct, and greatly enlarged. This piece of tin has been perforated for the purpose; place it close to your eye, and this small print before it.
Caroline. Astonishing! the letters appear ten times as large as they do without it: I cannot conceive how this effect is produced.