OW wondrous is the power of the eye! The immense expanse of sky and oceanthe crowds and buildings of a city-the woods and hills and streams of a landscape-are all seen by it with the same distinctness with which it marks the forms and hues and dimensions of the minute flower or insect. It is the most valuable of our organs of sense, and our admiration of the wondrous adaptations of the universe we inhabit is increased by the fact that light is provided for this organ. Through the aid of light, the figure of any object is pictured on the retina of the eye, just in proportion to its distance. It is upon what philosophers call the "incidence of light" that our vision thus depends.

From the extremities of every object rays are carried to the centre of the human pupil, or sight portion of the eye-ball. Now, lines drawn from the extremities of an object to the eye must describe a triangle, of which the object forms one side. The more distant the object, the longer must the sides of the triangle be, and the smaller the object will appear; the nearer it is, the shorter will be the sides, the greater will be the angle, and the larger will be the apparent size of the objects observed.

It is on this principle that both miscroscopes and telescopes

are formed. By the intervention of a lens, or of a convex glass, it was perceived that the rays of light passed through it were made to converge, through the concentrating power of the glass, very rapidly to a given point. If these rays converge to the inside, they, of course, diverge towards the outside. If a lens or a spherical piece of glass, therefore, be placed between the eye and any object within a short distance, the natural result will be that large angular rays will be formed between it and the eyes of the observer, and thus all its parts will be greatly magnified in appearance. These rays are called pencils, not only because they draw the object on the eye, but also from their figure.

This is the peculiarity of the microscope; and the next object to be attained in its construction, after the magnifying power, is the clearness with which it can be perceived. The principle is much the same in the telescope; and the chief difference to be observed in the two instruments appears to be that in the microscope the main object is to obtain intensity at a short distance, and in the telescope the cumulation of power through a long focal length: that is, to make the distance between the eye of the observer and the place where the rays would diverge to either side of the object, as long as possible. For the longer the focal length, the greater will be the cumulative and penetrative power of the glass; and the introduction of several glasses, as is the case in the best instruments, is only made for the purpose of strengthening the effect.

The inventor of the microscope is utterly unknown. That the ancients made some powerful applications of the lens is evident from the account given by Lucian and Galen that Archimedes burned the Roman fleet, at the siege of Syracuse, by means of glasses, two hundred and twelve years before our Yet neither the Greeks nor Romans have left us any account of the lens being applied to increase the stores of discovery in natural science; and the only authentic records.

era.

we have respecting the microscope belong to its later improvements.

The most useful lenses employed have always been made of glass; though for some time it was believed that precious stones, from their greater refractive power, would make the better lenses. But it was discovered that the substance of precious stones caused such an aberration of the rays of light, that no sufficiently definite object could be observed, and they were consequently disused. It may be briefly observed, that the chief object of a microscope is attained by the disposition of a number of the glass lenses, and the difficulty lies in concentrating the rays of light upon the object-glass.

This

This was accomplished in a very considerable degree by Dr. Wollaston, who invented what is called his doublet. consisted of two lenses, having one side flat and the other convex, with two pieces of flat glass placed between them. The two lenses were placed so as to present their flat sides towards the object. To this there were several objections; but it was still very superior to the single lens, and will transmit a pencil of light from 35 to 50 degrees, without any very perceptible errors. Dr. Wollaston was led to this discovery by the "eye-piece" constructed for the purpose of obtaining a distinct view of the heavenly bodies by the celebrated philosopher Huygens, by whom it was applied to the telescope only, which had the effect of preventing the appearance of those rays which so often tend to confuse the aspect of any luminous bodies, and which exhibits many objects very beautifully.

The next improvement was made by a gentleman of the name of Holland, who to the lens nearest the object added a smaller one, which had the effect of still further correcting the aberration and concentrating the sight on the real object to be observed. But one misfortune attended this increase of the number of lenses: they necessarily absorbed a portion of the

light, and it was soon perceived that three lenses were as many as could be used in the construction of a simple microscope.

Some advantage was obtained by the use of Dr. Wollaston's periscopic lens. This consisted of two hemispheric lenses connected together by their flat faces, an aperture being made between them, which was filled with opaque matter; and Mr. Coddington effected the same object in a better manner, by hollowing out a space of a complete sphere, and then filling it up as Dr. Wollaston had previously done.

But in the construction of a microscope the light under which the object is seen is almost of as much importance as the magnifying power itself; and in the investigation of every unknown substance, it ought to be placed in every possible position to receive the strongest luminence. It should be observed both wet and dry, immersed in such fluids as are best adapted to show its texture, such as water, alcohol, oil, and Canada balsam, which last has itself a power of refraction almost equal to that of glass. In some cases even it will be necessary to place the object between two glasses, and gently heat it to bring out the finer colours and fibres, and in this way the spiral vessels of asparagus and other similar vegetables may be very beautifully displayed.

The simple microscope is occasionally formed with three lenses, but a great improvement was effected when what is called the compound microscope was invented. In the latter instrument there are only two lenses, but they are so disposed as to give a cumulative magnitude to the object submitted for inspection. The first lens gathers the rays of light, and presents the object in its apparently increased size, and the second lens then magnifies the reflected object as if it were the original one, preserving all its power, and losing seemingly little by aberration; but yet the aberration and the confusion about the object was sufficient to be an important disadvantage, and for more than a century the compound microscope

remained without any improvement. But within the last fifteen years such improvements have been made in the compound microscope as have elevated it to the position of one of the most important instruments that has ever been applied to the promotion of human welfare.

About the year 1820 M. Sallignes in France began a series of experiments for the construction of what is called an achromatic object-glass. M. Chevalier in Paris, Signor Amici at Modena, Herr Frauenhofer at Munich, and Mr. Tulley in London, were at the same time engaged in a similar series of practical operations, and the latter gentleman, without knowing what had been effected on the Continent, succeeded in the construction of an instrument in which the achromatic objectglass, of nine-tenths of an inch focal length, had eighteen degrees of pencil.

Mr. Tulley was the first person in England who attained this object, and he succeeded in making a lens that would bear an eye-piece fitted to produce a magnifying power of one hundred and twenty. He afterwards invented a combination to be placed before that last mentioned, and which increased the angle of the pencil to thirty-eight degrees. He thus obtained a magnifying power of three hundred; and his glass, so far as accurate correctness of the field goes, has not been excelled by any subsequent invention.

While these several improvements were being made by the men practically engaged in the manufacture of philosophical apparatus, the attention of the most eminent persons engaged in the study of the pure sciences was assiduously turned to the same object. Professor Barlow, Mr. Coddington, Professor Airy, and Sir John Herschel, did much towards unfolding the true mathematical principles of the action of light and of the visual organs.

But the individual who was most successful in developing the abstruse doctrines on which the compound microscope was