rical form has the further advantage of rendering the position in which it is held of comparatively little consequence. It is therefore very convenient as a hand-lens, but its definition is of course not so good as that of a well-mado doublet or achromatic lens.
Another very useful form of doublet was proposed by Sir John Herschel, chiefly like the Coddington lens, for the sake of a wide field, and chiefly to be used iu the hand. It is shown in fee 0 : it consists of n double convex or crossed lens, having the nide of curvature as 1 to 6, and of a plane concave lens whose focal length is to that of tho convex lens as 13 to 5.
Various, Indeed innumerable, other forms and combinations of lenses have been"projected, some displaying much ingenuity, but few of any practical use. In the Catadioptrie lenses the light emerge" at right angles from its entering direction, being reflected from a surface cut at an angle of 45 degrees to the axes of the curved surfaces.
It was at one time hoped, as the precious stones are more refractive than glass, and as the increased refractive power is unaccompanied by a correspondent increase in chromatic dispersion, that they would furnish valuable materials for lenses, inasmuch as the refractions would be accomplished by shallower curves, and consequently with diminished spherical aberration. But these hopes were disappointed : everything that ingenuity and perseverance could accomplish was tried by Mr. Varley and Mr. Pritchard, under the patronage of Dr. Goring. It appeared, however, that the great reflective power, the doubly-refract ing property, the colour, and the heterogeneous structure of the jewels which were tried, much more than counterbalanced the benefits arising from their greater refractive power, and left no doubt of the superiority of skilfully made glass doublets and triplets. The idea is now, in fact, abandoned ; and the same remark is applicable to the attempts at constructing fluid lenses, and to the projects for giving to glass other than spherical surfaces.
By the term simple microscope is meant one in which the object is viewed directly through a lens or combination of lenses, just as we have aupposed an arrow or an insect to be viewed through a glass held in the hand. When, however, the magnifying power of the glass is considerable, in other words, when its focal length is very short, and its proper distance from its object of consequence equally short, it requires to be placed at that proper distance with great precision : it cannot therefore be held with eufficient accuracy and steadiness by the unassisted hand, but must be mounted in a frame havhig a rack or screw to move it towards or from another frame or stage which holds the object. It is then called a microscope, and it is furnished,
according to circumstances, with lenses and mirrors to collect and reflect the light upon the object, and with other conveniences which will now be described.
One of the best forms of a stand for a simple microscope is shown in fig. 10, where ? is a brass pillar serswed to a tripod base ; s is a broad stage for tho objects, secured to the stem by screws, whose milled primary object, a joint may be made at the lower end of the stem A, to allow the whole to take an inclined position ; and then the spring clips shown upon the stage are useful to retain the object in its place. Numerous convenient appendages may be made to accompany such microscopes, which it will be impossible to mention in detail : the most useful are Mr. Varley'a capillary cages for containing animalcule in water, and parts of aquatic plants ; also his tubes for obtaining and separating such objects, and his phial and phial-holder for preserving and exhibiting small living specimens of the Chace, Nitella, and other similar plants, and observing their circulation. The phial-microscope affords facilities for observing the operations of minute vegetable and animal life.
The mode of illuminating objects is one on which we must give some further information, for the manner in which an object is lighted is second in importance only to the excellence of the glass through which it is seen. In investigating any new or unknown specimen, it should be viewed in turns by every description of light, direct and oblique, as a transparent object and as an opaque object, with strong and with faint light, with large angular pencils and with small angular pencils thrown in all possible directions. Every change will probably develope some new fact in reference to the structure of the object, which should itself be varied in the mode of mounting iu *every possible way. It should be seen both wet and dry, and immersed in fluids of various qualities and densities, such, for instance, as water, alcohol, oil, and Canada balsam, which last has a refractive power nearly equal to that of glass. If the object be delicate vegetable tissue, it will be in some respects rendered more visible by gentle heating or scorching by a clear fire placed between two plates of glass. In this way the spiral vessels of asparagus and other similar vegetables may be beautifully displayed. Dyeing the objects in tincture of iodine will in some cases answer this purpose better.