Having cut a satisfactory section, let us proceed to describe the method of making a slide thereof. We will suppose that we do not wish to make a permanent preparation but one for temporary use. A clean slide must be selected, all through our pages we emphasise the cleanliness of slides, at or near its centre we put a drop of water and, lifting it with a brush, we place our section in the drop of water. If our examination is to be with a low magnification, we need not use a cover slip, nevertheless it is worth while to cultivate the habit of using one. The cover slip is not only a protection for our object but for our objective. Water you may argue cannot harm the lenses of the objective. Perhaps not, we will not argue the point but, when the water dries on the objective, it leaves a certain amount of deposit on the glass and this deposit must be rubbed off. The less often the lenses are rubbed the better for them, glass especially very highly polished optical glass, is far more easily scratched than many people imagine and a scratched lens is an inefficient lens. When using high magnifications a cover glass should always cover our object and the same remark applies to objects examined in Canada Balsam. This substance is likely to cause serious trouble if it finds its way on to an objective. It must be removed, that is obvious, but it sets hard, it must not be scraped away for fear of damaging the optical glass and, as it is used to cement the lenses together, there is the great danger that any solvent used to remove the Balsam from the face of the objectives, may also dissolve their setting. Our digression may seem somewhat unnecessary, but the very great importance of keeping all chemicals and even water, from coming into contact with the lenses of our instrument cannot be insisted upon too strongly. In whatever substance we examine our object, water, glycerine or Balsam, there is a right and a wrong way of applying the cover slip. It must not be dropped or laid down flat upon the object, if we do this we shall certainly imprison a number of air bubbles and that must be avoided. One edge of the cover slip must be laid against the edge of the mountant, as the liquid used for mounting our object is called, then placing a needle beneath the cover slip it must be gently lowered into position. We shall now find that all air bubbles are driven out as the cover slip is lowered.