The simplest of all devices for forming an image is what is termed a "Pinhole Camera."

If we make a minute hole in a thin sheet of card or metal, and place this at one end of a [rectangular] light-tight chamber or camera, and place a screen or sensitive photographic plate at the opposite end, it is easy to see that any luminous or illuminated object lying in front of this camera will form an image upon the screen.

Fig. 2.

Every luminous point that contributes to make up the entire object emits, as we have seen, luminous rays in all directions, but only one very small pencil of light can pass from each separate point through the tiny hole. Each minute pencil passes through this pinhole in straight lines, and passes on until it is intercepted by the screen, where it is received as a tiny dot of light, determining its image. Thus the entire object forms a complete image upon the screen or sensitive plate, as illustrated in the figure. (See note at the end of chapter.)

Successful photographs have been obtained by this means in cases where extremely fine definition is not of prime importance, nor the requisite time of exposure (a few minutes in good light) likely to emphasise any defect in definition by movement of the object.

The enthusiastic photographer, having inadvertently left his lens at home, has before to-day fixed his visiting card over the flange in his camera, pierced a hole in the pasteboard with a pin, and by this means secured a photograph that otherwise might never have been his, or fallen to his lot again.

FlG.3.

The "Pinhole Camera" gives us a clear insight into the formation of images of different sizes, and will help us to understand similar effects when we come to examine the capabilities of lenses in this respect.

First let us direct the camera towards an object situated at a given distance from the pinhole, the distance between the pinhole and the screen being also known.

Let us suppose the object ab (Fig. 3) to be 10 ft. distant from the pinhole p, and that p is 6 inches from the camera screen s; it is evident that the length of the image a b formed upon the screen bears the same proportion to the length of the object a b as the distance p s (6 inches) is to p o (10 ft.), or the image is one-twentieth of the size of the object.*

Now let us bring the camera nearer to the object, as shown in the same figure, until o p is only 5 ft., but o s remaining 6 inches. The image now formed upon the screen is affected in a similar manner to our impression of the appearance of the object. As we approach an object it appears to be larger, or the object is said to be viewed under a greater angle, and it now subtends a greater angle at the pinhole than it did in its first position. The rays passing from a and b through the pinhole cross at a greater angle. These and all rays forming the image are intercepted by the screen, and the length of the image is again determined by the relation of the distances po (5 ft.) and ps (6 inches), a b and a b being in the same proportion, or as 10 : 1.

Flg.4.

Thus we see that when the camera is one half the distance from the object, the image is double the size (linear); and in general the nearer we approach the object, the greater becomes the size of the image. By similar reasoning the converse may be taken for granted.

Let us now consider the case of the object and pinhole of the camera at a fixed distance apart, but with the screen of the camera made to occupy different positions.

* a p o and a p s are similar triangles, and if we know the relation existing between the measurement of any two similar sides, o p : p s :: 20 : 1, we know that any other pair of similar sides o a, as, bear the same relation to one another. Thus in the similar triangles a p b, a p b, a b : a b :: 20 : 1.

Let the object a b (Fig. 4) be again 10 ft. distant from r, and the screen s 6 inches from p, intercepting the rays of light from the object a b which pass through p, receiving the image at a b. In this position we have seen that the respective lengths of a b and a b are as 20 : 1, or as their distances from p.

Let us now move the screen nearer to p, as at p s', say 3 inches only from the pinhole. We observe that the luminous rays from ab are intercepted sooner, and form an image at a' b'. The relative lengths of ab and a'b' are now as 10 ft. to 3 inches, or as 40 : 1. Similarly, it is easy to see that if the screen be removed to s", at a distance of 12 inches from p, the image will now be intercepted at a" b", and that the relative lengths of a b and a" b" are now only as 10 : 1 ; and, in general, that the greater the distance between pinhole and screen, the larger will the image become.

FlG.5.

It will be seen later that when we employ an ordinary photographic lens, the only way to increase the size of the image upon the screen is by approaching the object, as we did with the pinhole camera in Fig. 3 ; but when we employ a Telephotographic lens, we can place the screen in any position we like, as in Fig. 4, and obtain different sizes of images from a fixed standpoint. So that the pinhole camera with the screen in a fixed position roughly defines the limited use of an ordinary photographic lens as regards its power of producing images of different size of a given object; whereas the pinhole camera with a movable screen indicates the far wider possiblities of the Telephotographic lens in this respect.