Concave lenses cause parallel rays to diverge as from a principal focus, which in these lenses is called a negative focus. For a plano-convex lens the negative focus will be at the distance of the diameter of the sphere; and for a double concave lens the centre of that sphere.

SPHERICAL AND CHROMATIC ABERRATION.

ON looking into some microscopes the edges of the object appear less distinct than its centre, or it may be surrounded by a rainbow-coloured ring.

The first of these defects is due to spherical aberration; the second to chromatic aberration.

SPHERICAL ABERRATION is caused by the central rays of light not arriving at a focus so soon as the peripheral rays.

A B, rays falling on the periphery of the lens; F, focus of these; a, b, rays
falling nearer the centre; f, more distant focus of these.

The remedy for this defect is to increase the curve of the central part of the lens, or to cut off the peripheral rays by the intervention of a diaphragm.

CHROMATIC ABERRATION depends not so much upon the form, as upon the material of which the lenses are made. Some substances cause white light to be separated into its component rays as in the common prism; and, as the different colours have different focal points, it will be found that according as the lens is focussed, the violet, blue, green, red, or yellow will prevail.

A B, rays of white light refracted by a convex lens ; c, the focus of the violet
rays, which then cross and diverge towards E F; D, the focus of the red
rays which are crossed at the points E E, by the violet; the middle point
of this line is the mean focus, or focus of least aberration.

To remedy this very serious defect two materials are employed, having different dispersive powers, so that one may neutralise the error produced by the other.

One substance having too much, and the other too little dispersive power, the plus of the one, and the minus of the other, together give nil.

In constructing the better class of objectives a double convex lens of crown glass is neatly fitted into a plano-concave of flint glass; and to their surfaces is given such a degree of curvature that the excess of aberration in the one is compensated by the minus of the other.

Three sets of lenses so constructed are generally combined in a good objective, fig, 9, P, M, a.

SECTION OF THE ENGLISH ACHROMATIC COMBINATION.

Aberration is still further corrected in the eye-piece, which is composed of two lenses, one called the 'eye glass,' from being next the eye of the observer, and the other termed the 'field glass,' from being nearer the field.

The field glass is very important, as it not only tends to correct aberration, but also causes the rays of light from the object glass to converge, so that none, or very few of them, are lost; thus rendering the image more clear and distinct than it would otherwise be.

The Compound Microscope.

The mechanical part of the instrument will be understood by reference to the accompanying drawing.

The lenses (g h) are fixed in a tube (a) set in a solid frame (b), to which is attached a stage (c) on which the object may be securely placed. Arrangements are made by means of rack and pinion for bringing the tube near to, or removing it from the object plate or stage, both by the coarse and fine adjustments (de). For the illumination of transparent objects a mirror (ƒ) is placed under the stage upon a movable arm, which allows it to be readily adjusted.

Every part of the instrument should be firm, and vibrate equally; and all the movements should be smooth and free from catches or jerks.

All the necessary mechanism of the microscope is extremely simple, and easily and quickly used. The luxurious complications of the most expensive instruments are little valued by the worker in the laboratory to whom time is important.

Even the recent improvements in the object-glasses of high power, which are made to transmit large pencils of light through their large angular apertures, and thus enable the observer to distinguish very delicate objects, such as the lines upon Diatomaceæ, are not adapted for ordinary scientific research; certainly not for every-day physiological or pathological investigations; not only because their adjustment is, of necessity, extremely delicate, but for the further reason that their power of penetration is impaired by the arrangement which improves their power of definition.

The object-glasses are designated according to the focal distance of a single lens of the same magnifying power: thus a two-inch objective is understood to be a combination which has the magnifying power of a single lens whose focal point is two inches from the object, and so in reference to the other powers. The following table will convey an idea of the magnifying power of each object-glass with the different eye-pieces.

These measurements are linear, but, with a view to astonish the public, it is common to speak of the superficial enlargement, which is the square of the linear; thus, if the quarter-inch magnifies an object about 200 diameters, or 200 linears, its superficial measurement will be obviously the square of the 200 linear, viz. 40,000..

The magnifying power of the eye-piece used must be added to the magnifying power of the objective in estimating the enlargement of the image of an object.

By the aid of different eye-pieces an extensive range of magnifying power may be obtained; for example, the two-inch objective with a deep eye-piece will give the same amplification as the quarter objective with the ordinary eye-piece; and for certain observations, the combination of low objectives with deep eye-pieces is by some considered to be advantageous.

Binocular Microscope.

The employment of the additional tube and eye-piece with Wenham's prism is now so general that little need be said in its favour.

The peculiar stereoscopic effects produced, particularly with the low powers, give a natural appearance to objects, not attainable with the single microscope. At the same time the binocular arrangement in no way interferes with the use of the instrument as a single microscope.

The accompanying drawing, fig. 11, represents the microscope. recommended by Dr. Harley to his students. It is constructed on his own plan, and possesses the advantage of having two object glasses and various pieces of apparatus, so arranged as to be brought into use without a moment's loss of time. It is specially adapted to the requirements of the busy man.

The microscope as is here seen is fixed into the bottom of the mahogany box which forms at the same time the stand. Round it a groove is run to receive the lip of a glass shade. The eyepieces are supplied with shades (a, a) to protect the eyes.

These are a great comfort to the observer when he is using the instrument for any length of time.

At the end of the transverse arm (ƒ) is the box which contains