DIAGRAM. EXPLAINED.

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large one, and the image of the object to be viewed being formed between these two lenses, the rays proceeding from it are made again to converge, on passing through the upper lens or eye-glass, in the focus of which, a distinct and greatly magnified image of the object is seen.

I hope this drawing will make the affair still plainer. The first figure is a microscope, like this we have been examining: the second is a diagram, in which slanting lines show the course of the rays, and the refractions they sustain in passing through the different lenses.

The arrow a b, represents any small object, which you may suppose to be laid on the stage of the microscope, as at F. C is the objectglass, such as you have seen screwed on the instrument at the part marked O in the drawing. The rays proceeding from the object, on entering this small lens, are converged. They cross one another, and pass out of the lens diverging: if they met with no interruption, these rays would continue to rise up through the body of the microscope, and would form, in the upper part

170 TELESCOPES AND MICROSCOPES COMPARED.

scopes; in both we have one tube sliding within another, to adjust the focus to different eyes; in both we have an object-glass, filling one end of the tube as a window, to receive and transmit the rays proceeding from the object to other lenses, called eye-glasses, because they assist the eye, by obtaining for it a larger field of view, and still further magnifying the image. And, as there are reflecting telescopes, so there are also reflecting microscopes, in which the image of an object magnified by reflection from a concave mirror, is still further magnified, by being viewed through a double convex lens. Sir Isaac Newton was the inventor of this kind of microscope; it represents the object distinctly, but inverted. So great is the resemblance of structure, that a telescope may be changed into a microscope, by removing the object-glass to a greater distance from the eye-glass; but whenever the tube is fitted for taking a view of distant objects, it s a telescope; when it is adapted for taking a view of little things which

*Hutton, ii p. 108

CAMERA-OBSCURA.

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are near to us, it is a microscope. Only bear in mind that těle means far off, and mikros small, and you can never forget the distinction between names which so clearly express the different purposes for which these beautiful instruments are intended.

Although it is impossible for me to give you an idea of all the ingenious contrivances which have resulted from the discovery of a lens possessing the curious property of bringing together into one focus all the rays that pass through it, I am desirous of showing you another example.

I told you that the telescope and the microscope derive their names from Greek words, expressive of their peculiar properties. The machine we are now about to examine, has a Latin name, camera-obscura, or dark-chamber; because the objects it discloses must be seen in a darkened room, or in a box like this, which is a dark chamber in miniature. It is useful in various ways, and may assist us hereafter in explaining

172

BAPTISTA PORTA.

the structure of the eye; but just now I mean to confine myself to the entertainment it is capable of affording, by presenting faithful and lively images of objects, not only in their natural colours and proportions, but with their actual motions and varying expressions. The objects passing along a busy street, birds flying, or trees waving in the wind, may form, in the camera-obscura a succession of moving pictures, beyond the power of art to imitate.

The invention is ascribed to John Baptista Porta, a learned Neapolitan, who died in the year 1515. He was fond of the society of persons like himself, and received them at his house; he also studied the curious phenomena of nature; and, as he lived in an age of great ignorance and superstition, he was accused of practising magical arts, and drew upon himself the censures of the Romish church.* Wbether he was led by some fortunate accident to contrive this beautiful instrument, I cannot tell you; but it is highly probable, as you may judge

Lempriere, Univ. Biog.