Wireless Transmission of Photographs Part 7

Get the coating on the foil as thin as possible, and err on the side of over-exposure, for if the coating is thick and has been under-exposed, excessive was.h.i.+ng will dissolve the whole coating; for, unless insolubilisation has taken place right up to the metal base, the under parts will remain in a more or less soluble condition.

On no account must the unexposed sheets be placed near a fire, otherwise they will be spoilt, the whole coating becoming insoluble; heat acting in the same manner as light.

In was.h.i.+ng, keep the print moving so that the stream of water does not fall continually in one place. It is best to hold the print so that the water runs off in the direction of the lines.

To dry the prints after was.h.i.+ng they can be laid out flat in a moderately warm oven, or before a stove, the heat of course not being sufficient to cause the coating to peel.

To render the glue image more distinct the print should be immersed for a few seconds in an aniline dye solution, the glue taking up the colour readily. These dyes are soluble in either water or alcohol. A dye known as "magenta" is very good.

The process of coating the metal sheets must be performed as quickly as possible (about 10 seconds), as owing to the peculiar nature of the b.i.+.c.hromated glue it soon sets, and once this has taken place it is impossible to smooth down any unevenness.

See that the negative and metal sheet make good contact while printing.

If the glue solution does not adhere to the surface of the foil in a perfectly even film, but a.s.sumes a streaky appearance, a little liquid ammonia, or a weak solution of nitric acid, rubbed over the surface of the foil, which is afterwards gently scoured with precipitated chalk on a tuft of cotton {124} wool, will remove the grease which is the cause of the difficulty.

A photograph of a picture prepared from a line negative is given in Fig.

61. For a great many experiments, and in order to save time, trouble, and expense, sketches drawn upon stout lead-foil in an insulating ink will answer the purpose admirably, but if any exact work is to be done a single line print is of course absolutely necessary. The insulating ink can be prepared by dissolving sh.e.l.lac in methylated spirit, or ordinary gum can be used. A very fine brush should be used in place of a pen, as the gum will not flow freely from an ordinary nib unless greater pressure than the foil will safely stand be applied. A sketch prepared in this manner is shown in Fig. 62. A little aniline dye should be added to the gum to render it more visible, or a mixture of gum and liquid indian ink will be found suitable.

[Ill.u.s.tration: FIG. 63.]

With the copying arrangement already described it is only possible to employ it for reducing, it being necessary to employ a bellows camera with a back focussing attachment for purposes of enlarging, and this const.i.tutes the chief drawback to the use of a fixed focus camera. By replacing the box camera with a focussing camera of the same size, we shall have a piece of apparatus capable of reducing or enlarging, only in this case the camera should be a fixture and the board, A, arranged to slide backwards and forwards instead.

[Ill.u.s.tration: FIG. 61.

Portions of photographs (full size) of single line screen, and single line print. Screen 40 lines to the inch.]

[Ill.u.s.tration: FIG. 62.]

{125} An extra improvement would be to rule the surface of the copying board, A, in a manner similar to that shown in the diagram, Fig. 63. The rulings should be marked off from the centre of the board, and should enclose parallelograms of the various plate sizes ranging from 3-1/4 4-1/4 inches up to the full size of the board. By fastening the picture or photograph to be copied in the s.p.a.ce on the board corresponding in size, we can ensure that it is in the correct position for the whole to be included on the photographic plate, providing, of course, that the centre of lens and board coincide.

With regard to the lens required, the practice adhered to by most photographers is to use a lens having a focal length equal to the diagonal of the plate used. Thus for a 1/4-plate camera a 5-inch lens should be used, and for a 1/2-plate an 8-inch lens, and so on. For a 5 4 inch camera a 6-inch lens will be required. The following is a simple rule for finding the conjugate foci of a lens, and is useful in obtaining the distance from the lens to the photographic plate and the picture to be copied. Let us suppose that we wish to make a 1-1/2 times enlarged line negative from a 4-1/4 3-1/4 inch print. Add 1 to the number of times it is required to enlarge and multiply the result by the focal length of the lens in inches. In the present case this will be 1-1/2 + 1 = 2-1/2; and if a 6-inch lens is used, 2-1/2 6 = 15 inches will be the distance of the lens from the plate. Divide this number by the number of times it is desired to enlarge, and the distance of the lens from the picture to be copied is obtained; in this instance 15 1-1/2 = 10 inches. The same rule can be followed when it is required to reduce any given number of times, only in this case the greater number will represent the distance between the lens and the picture to be copied, and the lesser number the distance between the lens and the plate.

In reducing, a 1/4-plate lens will be found to fully cover a 5 4 inch plate, providing the reduction is not greater than three to one.

{126}

APPENDIX C

LENSES

In this small volume it is not desirable, neither is it intended, to give an exhaustive treatment on the subject of lenses and their action, but as optics plays an important part in the transmission of photographs, both by wireless and over ordinary conductors, the following notes relating to a few necessary principles have been included as likely to prove of interest.

Light always travels in straight lines when in a medium of uniform density, such as water, air, gla.s.s, etc., but on pa.s.sing from one medium to another, such as from air to water, or air to gla.s.s, the direction of the light rays is changed, or, to use the correct term, _refracted_. This refraction of the rays of light only takes place when the incident rays are pa.s.sed obliquely; if the incident rays are perpendicular to the surface separating the two media they are not refracted, but continue their course in a straight line.

All liquid and solid bodies that are sufficiently transparent to allow light rays to pa.s.s through them possess the power of bending or refracting the rays, the degree of refraction, as already explained, depending upon the nature of the body.

The law relating to refraction will perhaps be better understood by means of the following diagram. In Fig. 64 let the line AB represent the surface of a vessel of water. The line CD, which is perpendicular to the surface of the {127} water, is termed the _normal_, and a ray of light pa.s.sed in this direction will continue in a straight line to the point E. If, however, the ray is pa.s.sed in an oblique direction, such as ND, it will be seen that the ray is bent or refracted in the direction DM; if the ray of light is pa.s.sed in any other oblique direction, such as JD, the refracted ray will be in the direction DK. The angle NDC is called the _angle of incidence_ and MDE the _angle of refraction_. If we measure accurately the line NC, we shall find that it is 1-1/3, or more exactly 1.336, times greater than the line EM. If we repeat this measurement with the lines JH and PK we shall find that the line JH also bears the proportion of 1.336 to the line PK. The line NC is called the _sine of the angle of incidence_ NDC, and EM the _sine of the angle of refraction_ MDE.

[Ill.u.s.tration: FIG. 64.]

Therefore in water the sine of the angle of incidence is to the sine of the angle of refraction as 1.336 is to 1, and this is true whatever the position of the incident ray with respect to the surface of the water. From this we say that _the sines of the angles of incidence and refraction have a constant proportion or ratio to one another_.

The number 1.336 is termed the _refractive index_, or _coefficient_, or the _refractive power_ of water. The refractive power varies, however, with other fluids and solids, and a complete table will be found in any good work on optics.

Gla.s.s is the substance most commonly used for refracting the rays of light in optical work, the gla.s.s being worked up into different forms according to the purpose for which it {128} is intended. Solids formed in this way are termed _lenses_. A lens can be defined as a transparent medium which, owing to the curvature of its surfaces, is capable of converging or diverging the rays of light pa.s.sed through it. According to its curvature it is either spherical, cylindrical, elliptical, or parabolic. The lenses used in optics are always exclusively spherical, the gla.s.s used in their construction being either crown gla.s.s, which is free from lead, or flint gla.s.s, which contains lead and is more refractive than crown gla.s.s. The refractive power of crown gla.s.s is from 1.534 to 1.525, and of flint gla.s.s from 1.625 to 1.590. Spherical surfaces in combination with each other or with plane surfaces give rise to six different forms of lenses, sections of which are given in Fig. 65.

[Ill.u.s.tration: FIG. 65.]

All lenses can be divided into two cla.s.ses, convex or converging, or concave or diverging. In the figure, _b_, _c_, _g_ are converging lenses, being thicker at the middle than at the borders, and _d_, _e_, _f_, which are thinner at the middle, being diverging lenses. The lenses _e_ and _g_ are also termed meniscus lenses, and _a_ represents a prism. The line XY is the axis or _normal_ of these lenses to which their plane surfaces are perpendicular.

Let us first of all notice the action of a ray of light when pa.s.sed through a prism. The prism, Fig. 66, is represented by the triangle BBB, and the incident ray by the line TA. {129} Where it enters the prism at A its direction is changed and it is bent or refracted towards the base of the prism, or towards the normal, this being always the case when light pa.s.ses from a rare medium to a dense one, and where the light leaves the opposite face of the prism at D it is again refracted, but away from the normal in an opposite direction to the incident ray, since it is pa.s.sing from a dense to a rare medium. The line DP is called the _emergent_ or refracted ray. If the eye is placed at T, and a bright object at P, the object is seen not at P, but at the point H, since the eye cannot follow the course taken by the refracted rays. In other words, objects viewed through a prism always appear deflected towards its summit.

[Ill.u.s.tration: FIG. 66.]

In considering the action of a lens we can regard any lens as being built up of a number of prisms with curved faces in contact. Such a lens is shown in Fig. 67, the light rays being refracted towards the base of the prisms or towards the normal, as already explained; while the top half of the lens will refract all the light downwards, the bottom half will act as a series of inverted prisms and refract all the light upwards.

[Ill.u.s.tration: FIG. 67.]

[Ill.u.s.tration: FIG. 68.]

If a beam of parallel light--such as light from the sun--be pa.s.sed through a double convex lens L, Fig. 68, we shall find that the rays have been refracted from their parallel course and brought together at a point F.

This point F is {130} termed the princ.i.p.al focus of the lens, and its distance from the lens is known as the focal length of that lens. In a double and equally convex lens of gla.s.s the focal length is equal to the radius of the spherical surfaces of the lens. If the lens is a plano-convex the focal length is twice the radius of its spherical surfaces. If the lens is unequally convex the focal length is found by the following rule: multiply the two radii of its surfaces and divide twice that product by the sum of the two radii, and the quotient will {131} be the focal length required. Conversely, by placing a source of light at the point F the rays will be projected in a parallel beam the same diameter as the lens. If, however, instead of being parallel, the rays proceed from a point farther from the lens than the princ.i.p.al focus, as at A, Fig. 69, they are termed divergent rays, but they also will be brought to a focus at the other side of the lens at the point a. If the source of light A is moved nearer to the princ.i.p.al focus of the lens to a point A^1 the rays will come to a focus at the point _a_^1, and similarly when the light is at A^2 the rays will come to a focus at the point _a_^2. It can be found by direct experiment that the distance _fa_ increases in the same proportion as AF diminishes, and diminishes in the same proportion as AF increases. The relations.h.i.+p which exists between pairs of points in this manner is termed the _conjugate foci_ of a lens, and though every lens has only one princ.i.p.al focus, yet its conjugate foci are innumerable.

[Ill.u.s.tration: FIG. 69.]

The formation of an image of some distant object in its princ.i.p.al focus is one of the most useful properties of a convex lens, and it is this property that forms the basis of several well-known optical instruments, including the camera, telescope, microscope, etc.

If we take an oblong wooden box, AA, and subst.i.tute a sheet of ground gla.s.s, C, for one end, and drill a small pinhole, H, in the centre of the other end opposite the {132} gla.s.s plate, we shall find that a tolerably good image of any object placed in front of the box will be formed upon the gla.s.s plate. The light rays from all points of the object, BD, Fig. 70, will pa.s.s straight through the hole H, and illuminate the ground gla.s.s screen at points immediately opposite them, forming a faint inverted image of the object BD. The purpose of the hole H is to prevent the rays from any one point of the object from falling upon any other point on the gla.s.s screen than the point immediately opposite to it, therefore the smaller we make H, the more distinct will be the image obtained. Reducing the size of H in order to produce a more distinct image has the effect of causing the image to become very faint, as the smaller the hole in H, the smaller the number of rays that can pa.s.s through from any point of the object. By enlarging the hole H gradually, the image will become more and more indistinct until such a size is reached that it disappears altogether.

[Ill.u.s.tration: FIG. 70.]

If in this enlarged hole we place a double convex lens, LL, Fig. 71, whose focal length suits the length of the box, the image produced will be brighter and more distinct than that formed by the aperture, H, since the rays which proceed from any point of the object will be brought by the lens to a focus on the gla.s.s screen, forming a bright {133} distinct image of the point from which they come. The image owes its increased distinctness to the fact that the rays from any one point of the object cannot interfere with the rays from any other point, and its increased brightness to the great number of rays that are collected by the lens from each point of the object and focussed in the corresponding point of the image. It will be evident from a study of Fig. 71 that the image formed by a convex lens must necessarily be inverted, since it is impossible for the rays from the end, M, of the object to be carried by refraction to the upper end of the image at _n_. The relative positions of the object and image when placed at different distances from the lens are exactly the same as the conjugate foci of light rays as shown in Fig. 69.

[Ill.u.s.tration: FIG. 71.]

The length of the image formed by a convex lens is to the length of the object as the distance of the image is to the distance of the object from the lens. For example, if a lens having a focal length of 12 inches is placed at a distance of 1000 feet from some object, then the size of the image will be to that of the object as 12 inches to 1000 feet, or 1000 times smaller than the object; and if the length of the object is 500 inches, then the length of the image will be the 1/1000th part of 500 inches, or 1/2 inch. {134}

The image formed by the convex lens in Fig. 71 is known as a _real image_, but in addition convex lenses possess the property of forming what are termed _virtual images_. The distinction can be expressed by saying, _real images are those formed by the refracted rays themselves, and virtual images those formed by their prolongations_. While a real image formed by a convex lens is always inverted and smaller than the object, the virtual image is always erect and larger than the object. The power possessed by convex lenses of forming virtual images is made use of in that useful but common piece of apparatus known as a reading or magnifying gla.s.s, by which objects placed within its focus are made larger or magnified when viewed through it; but in order to properly understand how objects seem to be brought nearer and apparently increased in size, we must first of all understand what is meant by the expression, _the apparent magnitude of objects_.

[Ill.u.s.tration: FIG. 72.]

The apparent magnitude of an object depends upon the angle which it subtends to the eye of the observer. The image at A, Fig. 72, presents a smaller angle to the eye than the angle presented by the object when moved to B, and the image therefore appears smaller. When the object is moved to either B or C, it is viewed under a much {135} greater angle, causing the image to appear much larger. If we take a watch or other small circular object and place it at A, which we will suppose is a distance of 50 yards, we shall find that it will be only visible as a circular object, and its apparent magnitude or the angle under which it is viewed is then stated to be very small. If the object is now moved to the point B, which is only 5 feet from the eye, its apparent magnitude will be found to have increased to such an extent that we can distinguish not only its shape, but also some of the marking. When moved to within a few inches from the eye as at C, we see it under an angle so great that all the detail can be distinctly seen.

By having brought the object nearer the eye, thus rendering all its parts clearly visible, we have actually magnified it, or made it appear larger, although its actual size remains exactly the same. When the distance between the object and the observer is known, the apparent magnitude of the object varies inversely as the distance from the observer.

Let us suppose that we wish to produce an image of a tree situated at a distance of 5000 feet. At this distance the light rays from the tree will be nearly parallel, so that if a lens having a focal length of 5 feet is fastened in any convenient manner in the wall of a darkened room the image will be formed 5 feet behind the lens at its princ.i.p.al focus. If a screen of white cardboard be placed at this point we shall find that a small but inverted image of the tree will be focussed upon it. As the distance of the object is 5000 feet, and as the size of the received image is in proportion to this distance divided by the focal length of the lens, the image will be as 5000 5, or 1000 times smaller than the object.