Part 15 (1/2)

A lens which has been corrected for colour is still ih all parts of it, those which strike it near the edge will be refracted more than those near the centre, and a blurred focus results This is termed _spherical aberration_ You will be able to understand the reason fros 113 and 114 Two rays, A, are parallel to the axis and enter the lens near the centre (Fig 113) These meet in one plane Two other rays, B, strike the lens very obliquely near the edge, and on that account are both turned sharply upwards, co to a focus in a plane nearer the lens than A If this happened in a camera the results would be very bad Either A or B would be out of focus The trouble isin front of the lens a plate with a central circular opening in it (denoted by the thick, dark line in Fig

114) The rays B of Fig 113 are stopped by this plate, which is therefore called a _stop_ But other rays froh the hole These, however, strike the lens much more squarely above the centre, and are not unduly refracted, so that they are brought to a focus in the same plane as rays A

[Illustration: FIG 113]

[Illustration: FIG 114]

DISTORTION OF IMAGE

[Illustration: FIG 115--Section of a rectilinear lens]

The lens we have been considering is a single raphy, mounted with the convex side turned towards the inside of the ca the stop in front of it If you possess a lens of this sort, try the following experie square on a sheet of white paper and focus it on the screen The sides instead of being straight boards: this is called _barrel_ distortion Now turn the lens mount round so that the lens is outwards and the stop inwards The sides of the square will appear to boards the centre: this is _pin-cushi+on_ distortion For a long tie S Cundell suggested that _two_ meniscus lenses should be used in co 115 Each produces distortion, but it is counteracted by the opposite distortion of the other, and a square is represented as a square Lenses of this kind are called _rectilinear_, or straight-line producing

We have now reviewed the three chief defects of a lens--chromatic aberration, spherical aberration, and distortion--and have seen how they may be remedied So ill now pass on to the most perfect of ca 116) is nearly spherical in form, and is surrounded outside, except in front, by a hard, horny coat called the _sclerotica_ (S) In front is the _cornea_ (A), which bulges outwards, and acts as a transparentto adht to the lens of the eye (C) Inside the sclerotica, and next to it, coain is the _retina_, or curved focussing screen of the eye, whichfroht sensations to the brain The hollow of the ball is full of a jelly-like substance called the _vitreous humour_; and the cavity between the lens and the cornea is full of water

We have already seen that, in focussing, the distance between lens and ie depends on the distance between object and lens Now, the retina cannot be pushed nearer to or pulled further away fro screen of a camera How, then, is the eye able to focus sharply objects at distances varying from a foot to many miles?

[Illustration: FIG 116--Section of the human eye]

As a preliminary to the anse must observe that the more convex a lens is, the shorter is its focus We will suppose that we have a box caidly in the position necessary for obtaining a sharp ie of distant objects It so happens that ant to take with it a portrait of a person only a few feet from the lens If it were a bellows camera, we should rack out the back or front But we cannot do this here So we place in front of our lens a second convex lens which shortens its principal focus; so that _in effect_ the box has been racked out sufficiently

Nature, however, employs a much more perfect method than this The eye lens is plastic, like a piece of india-rubber Its edges are attached to ligaments (L L), which pull outwards and tend to flatten the curve of its surfaces The normal focus is for distant objects When we read a book the eye adapts itself to the work The ligaments relax and the lens decreases in dia at the centre, until its curvature is such as to focus all rays froh theat soaments pull on the lens envelope and flatten the curves

This wonderful lens is achromatic, and free froe Nor et that it is aided by an automatic ”stop,” the _iris_, the central hole of which is naray eyes according to the colour of the iris Like the lens, the iris adapts itself to all conditions, contracting when the light is strong, and opening when the light is weak, so that as uniforht as conditions allow may be admitted to the eye Most modern camera lenses are fitted with adjustable stops which can beon theseen is thrown on the retina upside down, and the brain reverses the position again, so that we get a correct is

THE USE OF SPECTACLES

[Illustration: FIG 117_a_]

[Illustration: FIG 117_b_]

[Illustration: FIG 118_a_]

[Illustration: FIG 118_b_]

The reader will now be able to understand without reat ht For one reason or another the distance between lens and retina becouish distant objects clearly The lens, as shown in Fig 117_a_, is too convex--has its minimum focus too short--and the rays eneral confusion of outline This defect is si 117_b_) a _concave_ lens, to disperse the rays somewhat before they enter the eye, so that they coht is thus corrected for distant objects, he can still see near objects quite plainly, as the lens will accommodate its convexity for theht, or _hypernifies that the eyeball is too short or the lens too flat Fig 118_a_ represents the nor at a distant object the eye thickens slightly and brings the focus forward into the retina But its thickening power in such an eye is very limited, and consequently the rays from a near object focus behind the retina It is therefore necessary for a long-sighted person to use _convex_ spectacles for reading the newspaper As seen in Fig 118_b_, the spectacle lens concentrates the rays before they enter the eye, and so does part of the eye's work for it

Returning for a116), we notice a black patch on the retina near the optic nerve This is the ”yellow spot” Vision is e of the object looked at is formed on this part of the retina The ”blind spot” is that point at which the optic nerve enters the retina, being so called fro of the blind spot is an interesting little experie and a shth, the other half an inch in dia the card near the face so that an eye is exactly opposite to each spot, and close the eye opposite to the smaller Now direct the other eye to this spot and you will find, if the card be moved backwards and forwards, that at a certain distance the large spot, though er than its fellow, has completely vanished, because the rays from it enter the open eye obliquely and fall on the ”blind spot”

Chapter XIII