Part 14 (2/2)

Fig 108 shows diagrae From A and B, the extremities of the object, a sih the centre of the lens This is not deflected at all Two other rays from the same points strike the lens above and below the centre respectively These are bent inwards and meet the central rays, or come to a focus with them at A1 and B1 In reality a countless number of rays would be transmitted froe

[Illustration: FIG 108--Showing how an ie is cast by a convex lens]

FOCUS

We must now take special notice of that word heard so often in photographic talk--”focus” What is th of a lens? Well, it nifies the distance between the optical centre of the lens and the plane in which the ie is forress a109 The object, O, in each case is assura froe to a focus at F If the distance between F and the centre of the lens is six inches, we say that the lens has a six-inch focal length The focal length of a lens is judged by the distance between lens and ie when the object is far away To avoid confusion, this focal length is known as the _principal_ focus, and is denoted by the syram the object is quite near the lens, which has to deal with rays striking its nearer surface at an acuter angle than before (reckoning from the centre) As the lens can only deflect their path to a fixed degree, they will not, after passing the lens, coether until they have reached a point, F1, further from the lens than F The nearer we approach O to the lens, the further away on the other side is the focal point, until a distance equal to that of F frolass in a parallel pencil The rays now coe If O be brought nearer than the focal distance, the rays would _diverge_ after passing through the lens

RELATIVE POSITIONS OF OBJECT AND IMAGE

[Illustration: FIG 110--Showing how the position of the ie alters relatively to the position of the object]

From what has been said above we deduce two ht to the lens, the further away froe be (2) If the object approaches within the principal focal distance of the lens, no ie will be cast by the lens To110), which shows five positions of an object and the relative positions of the ie (in dotted lines)

First, we note that the line A B, or A B1, denotes the principal focal length of the lens, and A C, or A C1, denotes twice the focal length We will take the positions in order:--

_Position I_ Object further away than 2_f_ Inverted i _f_

_Position II_ Object at distance = 2_f_ Inverted ie at distance = 2_f_, and of size equal to that of object

_Position III_ Object nearer than 2_f_ Inverted ier_ than the object

_Position IV_ Object at distance = _f_ As rays are parallel after passing the lens _no_ ie is cast

_Position V_ Object at distance less than _f_ No real i screen--is now given by the lens, but a e exists on the same side of the lens as the object

We shall refer to _virtual_ ith presently It is hoped that any reader who practises photography will now understand why it is necessary to rack his ca objects at close quarters Froather one practically useful hint--naram, etc, full size, both it and the plate must be exactly 2_f_ from the optical centre of the lens And it follows from this that the further he can rack his caeinal

CORRECTION OF LENSES FOR COLOUR

We have referred to the separation of the spectruht by a prism Now, a lens is one for 111 we assume that two parallel red rays and two parallel violet rays froh a lens A lens haseffect on violet rays and least on red, and the other colours of the spectrum are intermediately influenced For the sake of simplicity we have taken the two extremes only You observe that the point R, in which the red rays -point of the violet rays A photographer very seldom has to take a subject in which there are not objects of several different colours, and it is obvious that if he used a siood focus, the blue and green portions of his picture would necessarily be more or less out of focus

[Illustration: FIG 111]

[Illustration: FIG 112]

This defect can fortunately be corrected by the

112 A _colass convex elelass For the sake of illustration the two parts are shown separated; in practice they would be ce one optical body, thicker in the centre than at the edges--a meniscus lens in fact, since A is not so concave as B is convex Now, it was discovered by a Mr Hall h two silass, the for the spectrum colours--that is, violet rays were bent aside more suddenly colass prish the flint glass are but little deflected, while the violet rays turn suddenly outwards This is just what is wanted, for it counteracts the unequal inward refraction by B, and both sets of rays come to a focus in the same plane Such a lens is called _achrolass soe print you will see that the letters are edged with coloured bands, proving that the lens is not achroraphic lens would not show these pretty edgings Colour correction is necessary also for lenses used in telescopes and microscopes

SPHERICAL ABERRATION