DETERMINING POSITION OF STOPS.
The practical method of determining the distance from the lens at which to place the stop is to cut a length of cardboard tubing, and in it fix the lens by springing in the tube a wire ring to form a shoulder for the lens to rest against, and then an other ring to keep the lens squarely fixed in the tube. A slot must be cut along the tube at the bottom, and a card stop with a piece of wire attached placed in the tube flat against the lens. Focus a square sheet of type and it will be found to he sharp only in the centre. Move the stop slowly down the tube away from the lens by means of the wire, and note that the definition spreads rapidly towards the margins, but as the definition spreads the square begins to assume the appearance of a barrel. The best position is that giv ing the maximum of covering power with the minimum of distortion, usually where the stop just illuminates the corners of the plate. As this position is not always pos sible, the following method may be sub bottom of a small saucepan and place the lens, which must have been removed from its cell, upon it. To do this it may be
necessary to turn over the edge of the cell ; but they are often screwed in with an inner rim. Warm water is then poured gently into the saucepan until the lens is covered. The saucepan then is heated gently and kept so until the lenses are found to slide easily apart. The old bal sam may now be cleaned off with ether upon a silk handkerchief, but great care must be taken not to scratch the surfaces or injure the polish. Afterwards wash in stituted. Draw a square each side of which is equal to the focus of the lens, as F' Wig. 49), and a diagonal line II li, and from its centre erect a perpendicular J ie equal to the focus ; at At draw a line lc equal to the diameter of the lens. Now draw two lines from a and K through N and o until they meet at L. The distance from 31 to L is the distance required.