A solar beam of light is a bundle of rays; a ray being the smallest portion of light which can emanate from a luminous body.

Each of these rays possesses distinctive characters, both as regards their chemical functions and colors.

Sir Isaac Newton proved that the white light emitted by the sun is not so simple as it appears, but is composed of vivid colors, as shown by his beautiful experiment, the Analysis of Light, which is exemplified by the use of a glass prism. (See Fig. 1.)

The ray of light A, E, being admitted into a darkened room through a hole A, in the shutter, would fall upon the wall at E. As soon as the prism, B, C, is placed in the path of the sunbeam so as to allow it to fall on one of its angles B, the ray will be refracted, or bent out of its course so as to pass through the prism (as in the line D) and not in the same line, A, E, that it would have done had the prism not been interposed.

Another effect also takes place; the ray of white light is decomposed into its component colors, and if you stand at a short distance from the prism, you will see that these colors are spread out in a triangular form, the base of which is on the wall and the apex at the angle C of the prism. Remove the prism and it is seen that the splendid display of colors upon the wall has disappeared, and a round spot of white light, E, is seen below the place occupied by the spectrum.

The colored image on the wall is called the prismatic or solar spectrum, which, according to Sir Isaac Newton, is composed of seven different colors. The color at the lowest portion of the image is red and the one at the other end is violet, the intermediate parts being occupied by five other colors, and the whole arranged according to the table below, the proportion of each color having been measured by Fraunhofer with the greatest care, with the results placed opposite to each, corresponding with the 360 degrees of a circle, the red ray being the least and the violet the most refracted of this chromatic image.

Camera Lenses The Optics Of Photography PhotographyInStudioAndField 21

Fig. 1.

TOP.

Violet.......................

109

Indigo..........................

47

Blue..........................

48

Green........................

46

Yellow...........................

27

Orange.................................

27

Red.................................

56

360

The sunbeam, the ray of white light, contains powers within it of which the earlier philosophers had but a faint idea, besides its accompanying heat. There is a principle associated intimately with it, which has the power of decomposing and of determining the decomposition of chemical compounds.

This principle is "Actinism" and is as perfectly distinct in the nature of its properties from light, as light is from the principle of heat, with which it is also closely connected.

Actinism may then be considered as the fundamental principle, on which photography is based, and its power is exerted in forming the image on the sensitized plate in the Camera, as well as subsequently in forming or causing the impression on the sensitized paper exposed to the light beneath the negative.

In this connection we have only to consider the application of this Actinism through the medium of a combination of lenses to form an image on a sensitized plate in the Camera.

It has been shown that when light passes through a prism of glass its colors are separated; this separation is caused by the unequal refrangibility of the different colored rays, the violet being the most and the red the least refrangible of the seven rays.

A ray of light passing through a vacuum progresses in a perfectly straight line, but all matter, however attenuated it may be, has the property of refracting or bending the ray of light.

The refractive power of some substances is immense, while that of others is very trifling. The mode of the refraction depends on the comparative density or rarity of the respective media. If the medium which the rays enter be denser, they move through it in a direction nearer to the perpendicular drawn to its surface; on the contrary, when light passes out of a denser into a rarer medium, it moves in a direction farther from the perpendicular. This refraction is greater or less, that is the rays are more or less bent or turned from their straight course, as the second medium through which they pass is more or less dense than the first.

We next study the utilization of this power of refraction in the manufacture of lenses to overcome the unequal refrangibility of the colored rays of light.

A lens for use in photography is made of glass as pure and as colorless as can be procured, and is ground into such a form as to collect or disperse the rays of light which pass through it. Lenses are of different shapes, and thence receive different names.

Camera Lenses The Optics Of Photography PhotographyInStudioAndField 22

Fig. 2 E.

1. Double convex.

2. Plano-convex.

3. Concavo-convex.

4. Double concave.

5. Plano-concave.

6. Meniscus.

The figures 1 to 6 represent sections of the variously shaped lenses which are combined for use in photography.

The design in forming lenses is to procure a medium through which the rays of light from any object may pass and converge to a corresponding point beyond; the manner in which the rays proceed through the lens, and then centre in a focal point, will depend on the form of the lens, its capacity for refraction and the distance of the object.

The double convex lens may be viewed as a portion cut out of the side of a sphere. Here, as in all cases of convexity, the focus of the parallel rays passing through the lens is at the centre of the sphere. (See Fig. 3.)

A plano-convex has only half of the refractive power of the double convex; the parallel rays, falling on the convex side of the lens, would converge at the distance of the whole diameter of the sphere. Thus the focal point at which the rays of light converge is always regulated by the degree of curvature of the lens. Thus the double convex lens has the greatest power of converging the rays of light; the plano-convex has only half the power of the former. Both these lenses have also the power of magnifying the image of an object seen through them in the same proportion.