The great distinction between the simple and compound microscope is, that whereas the former instrument magnifies the object, the latter magnifies the magnified image of the object. In the least elaborate form of this instrument there are two glasses, one at each end of a tube, the small glass magnifying the object, and being therefore called the “object-glass,” while the other, which magnifies the image of the object, is placed next to the eye, and is therefore termed the “eye-glass.” In practice, however, this arrangement is found to be so extremely defective, that the instrument was quite useless except as an experimental toy; for the two enemies of the optician, chromatic and spherical aberration, prevailed so exceedingly, that every object appeared as if surrounded with prismatic colors, and every line was blurred and indistinct. In this uncertain state the compound microscope remained for many years, its superb capabilities being scarcely recognized. The chief fault was thought to be in the material of which the object-glass was made, and for a long series of years all experiments were conducted with a view to an improvement in this respect. When, however, the diamond had been employed as an object-glass, and had failed equally with those of less costly material, attention was directed to the right point--namely, the arrangement of the different glasses,--and at length opticians succeeded in obtaining a pitch of excellence which can be almost termed perfection. It would be impossible to describe the method which is employed for this purpose, and it must suffice to say that the principle is that of playing off one defect against another, and so making them mutually correct their errors. The magnifying powers of the compound microscope can be very great, and it is therefore necessary that extreme care should be taken in its manipulation. It will be possible for a clumsy person to do more damage to a good instrument in three minutes than can be repaired in as many weeks. Before proceeding to the management of the microscope and the construction of the “slides,” we will briefly describe one or two chief forms of the compound microscope.