We have already mentioned that the optical parts of the compound microscope are of greater importance than what may be termed the mechanical portions and the objectives are more important than the eyepieces. Better results can always be obtained with a good, high-power objective and a low-power eyepiece, than with an inferior objective and a good quality eyepiece. The merits of the eyepiece, however great, will not be adequate compensation for the failings of the objective. Modern objectives are composed of several lenses and of a combination of flint and crown glass, as we explained in our last chapter. They are so designed that they can be screwed into the lower part of the microscope tube. The focal length of each objective is, or should be, marked upon it; as a general rule, however, it may be taken that the smaller the lower lens, the shorter its focal length and therefore the greater its magnifying power. The form of eyepiece most usually met with is known as Huyghen’s. It consists of two plano-convex lenses, with their flat or plane surfaces directed away from the objective. The smaller of the two lenses is situated nearer to the eye of the observer and is known as the eyeglass; its function is to magnify the image formed by the objective. The larger, lower lens is known as the field or collecting glass; it renders the image clearer though, in so doing, it reduces the magnification of the eyeglass. In instruments provided with more than one eyepiece we shall wish to know which gives the greater magnification; this is or should be marked upon the metal rim surrounding the eyeglass but, in general, it may be stated that the shorter the eyepiece the greater its magnification. We repeat again, increase your magnification always, when possible, by using higher power objectives rather than eyepieces with greater magnifying powers. Sometimes it is necessary to use a greater magnification than our most powerful objective will give us; then we must fit our most powerful eyepiece and draw out the upper part of the microscope tube--in the best instruments they are made to pull out, after the manner of the telescope. The effect of so doing will be to increase the magnification considerably but, at the same time, the definition or clearness is seriously impaired.