10. _Section-Knife._—Of not less importance than the microtome is the section-knife, to be used in conjunction with it. How perfect soever the former, and whatever the dexterity of the operator, unless he be provided with a suitable and well-made knife, he will never succeed in obtaining satisfactory results. As to the most desirable _size_ of the knife, much difference of opinion seems to exist, section knives varying in this respect from a blade of extreme shortness to one which fell under our observation, in which the portentous length of _thirteen_ inches was attained. What advantages were to be expected by prolonging the blade to this extravagant length, must remain an inscrutable mystery to all save its designer. Concerning the _shape_ of the knife, it is frequently advised that the surface which has to glide along the cutting-plate of the microtome should be ground _flat_. A most unsuitable arrangement, as a very little actual experience of section-cutting will speedily demonstrate. After many unsuccessful attempts to obtain a really good and reliable section-knife, we determined to have one specially made, which, as it has proved everything that could be desired, merits a brief description. It is of the utmost importance that the blade be made of good and well-tempered steel, not only that it may be capable of receiving an edge of the most exquisite keenness, but also that it may _retain it_. The knife of which we speak (and which was made by Mr. Gardner, of South Bridge, Edinburgh) is furnished with a blade _four inches_ long, and 7/8-inch broad, set into a square handle of boxwood, also four inches in length. The thickness of the blade at the back is not quite 1/4-inch, while _both_ of its surfaces are slightly hollow ground. It is essentially necessary that the back and edge of the blade be strictly parallel to each other, otherwise the knife, when in use, will have such a tendency to tilt over as to render its management extremely difficult. It is very easy to discover if this condition be fulfilled, for if on carefully laying the flat of the blade upon a piece of level glass, every portion of both back and edge are found to be in close contact with it, the knife may in this respect be considered perfect. Every student who aspires to be a successful section-cutter should provide himself with a good Turkey oilstone, _and learn to use it_. He should also possess a razor strop, as it will be in constant requisition.[7] It may here be remarked that though _razors_, as a rule, are unsuitable for use with the microtome from want of uniformity in the thickness of their blades, yet, if only a small object is to be cut—for instance, a thin root or stem—very good results may be obtained from their use, especially if one of the old-fashioned make, having a thick back and slightly _concave_ surfaces, be employed.