When two lenses are required to be cemented together so as to present one common surface, they are slightly warmed, and a drop or two of Canada balsam is applied, and the two lenses pressed forcibly together, so as to squeeze out excess of balsam. When cooled, they present the appearance of one single piece of glass, and cannot be separated without heat. When two lenses have not a common surface, three small pieces of tinfoil are introduced at equal distances apart between their margins, or when the separation is greater, as in most portrait lenses, a ring of brass is used for the same purpose. When the lens is fixed in its brass ring, so that it cannot be taken out without raising the bent edge of the brass, it is said to be set.

Under the article FOCUS will be found numerous rules and tables, which may be of some service. The following, however, were not given then, and are, therefore, here introduced: —In portrait lenses, and some rapid rectilinears, when the latter are used at more than their equivalent focus, it is often desirable to know what depth of focus a lens possesses: depth of focus may be both before a given point and also behind it. The following rules are then required:—

Having focussed any point, to find the distance in front of that point which will be in focus (all measurements to be in inches, and the distance of object to be measured from the optical centre of lens)—

1. Multiply the focal length by the diameter of the stop, and the result by the difference between the focal length and the distance of the object.

2. Multiply the focal length by the diameter of the stop, and add 1/100 part of the distance of the object.

3. Divide the first product by the last, add the focal length, and subtract the result from the distance of the object, when the result will be the distance sought for in front in inches.

To find the depth of focus behind a given point—

1. Multiply the focal length by the diameter of the stop, and the result by the difference between the focal length and the distance of the object.

2. Multiply the focal length by the diameter of the stop, and subtract 1/100 part of the distance of the object.

3. Divide the first product by the last, add the focal length, and deduct the distance of the object; the result is the distance behind in inches.

Example. —Find the depth of focus when focussing an object 5 ft. distant with a lens of 7 in. focus, working at f/5.