Page images
PDF
EPUB

between its ends can be varied. By making this the proper distance the image is brought to a focus on the ground glass, then the sensitized plate is put in its place, and the exposure is made.

The plate is afterward developed by means of certain chemicals. This process reduces the silver salts in the film to a metallic condition, forming a dark layer wherever the light has acted. The salt that has not been acted upon by the light is dissolved out by putting the plate in a solution of hyposulphite of sodium, after which the plate is washed and dried. The plate is now called a negative, and prints can be made from it upon sensitized paper. (Figs. 537, 538.) A lantern slide is made from a negative by printing or photographing the negative upon a sensitized glass plate.

564. The Eye as an Optical Instrument. The eye is a minute camera, with dark chamber, lens, diaphragm, and

[merged small][ocr errors][merged small][merged small]

screen upon which the image is formed. The sclerotic coat S (Fig. 518) forms the wall of the dark chamber, and is extended in front as a transparent coat C called the cornea. The lens L of the eye is formed of an elastic, transparent substance, and is called the crystalline lens. Extending over the front of the lens is a colored curtain or diaphragm, the iris, which determines the color of the eye, and which has

[graphic][subsumed][merged small][graphic][merged small]

a circular opening in the center called the pupil. The image of an object is brought to a focus upon the inner lining r of the eye called the retina, from which the sensation of sight is carried to the brain by means of the optic nerve. The yellow spot Y is the most sensitive part of the retina, and, since it is on the axis of the eye, is the spot on which is formed the image of the point at which the eye looks directly.

We have seen that in the camera the distance between the lens and the ground glass is changed in order to bring a given object to a focus. The distance between the crystalline lens and the retina, however, is a fixed distance, and the focus is obtained by a change in the curvature of the front of the lens, thus changing its focal length. This change of curvature of the lens to change its focus for objects at different distances is called accommodation.

565. The Blind Spot. At the inner side of each eye where the optic nerve enters there is a blind spot. This can be readily proved as follows: Close the left eye and look steadily at the cross below with the right. A position can be found in which, while you look steadily at the cross,

+

O

the circle will disappear. When this position is found the circle may be brought into view by moving the book either nearer to the eye or farther away.

566. The Adaptability of the Retina of the eye as a screen for receiving the image formed - because of its concave shape is shown as follows:

Demonstration.

Fix a convex lens in the shutter of a dark Place a white paper screen in such a position that the middle of the image of the landscape will be in focus. The edges

room.

will be blurred and indistinct. of a section of the surface of an upright cylinder, and all parts of the picture on a horizontal line will be equally distinct. Turn the screen to other positions and observe the effect.

Now bend the screen into the form

567. Light and Illumination. The most important use of artificial light is the illumination of interiors. For interior

[graphic]

Opaque bowl

illumination both the incandescent electric lamp, preferably the tungsten, and the incandescent mantle gas lamp, provide light units that combine convenience and efficiency to a high degree.

There are three systems of distribution that are used with both gas and electric lighting, the direct, the indirect, and the semi-indirect.

The direct system is one in which the light from the lamp shines directly upon the area lighted, either with or without the help of reflecting shades. This system gives the maximum amount of light for a given expenditure of energy, but is objectionable on account of the glare of the light which either comes directly from the lamp or is reflected from polished surfaces. This objection can be partially overcome by the use of frosted lamps or translucent globes, when it becomes a modified direct system.

Interior reflectors

FIG. 539. Indirect System

[ocr errors]

The indirect system is one in which the lamps are placed within two or three feet of the ceiling while opaque, bowlshaped reflectors are placed below the lamps. The inner

surface of the bowls is made of a highly reflecting material and thus the light from the lamp is reflected from the ceiling, from which it is diffused throughout the room. Less light is thrown on a given surface, a table top, for example, for the same expenditure of energy, than in the direct system, but as the eyes are not tired by the direct glare of the light, objects are seen equally well under the reduced intensity. This system does away with the harsh shadows of the direct system, the only shadows formed being soft and pleasing. One form of bowl with interior reflectors is shown in Fig. 539. The semi-indirect system is one which is intended to com

[graphic][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

bine the advantages of both the direct and indirect systems and to avoid the objectionable features of each. The method adopted is to reflect a part of the light to the ceiling for general illumination and to direct the rest downward either directly or through translucent screens.

This system has the advantages of lighting a limited area well without concentrating all the light upon it and leaving the rest of the room unlighted.

In order to light a limited area, such as the top of a desk. or table, reflecting shades are used. These are of various types, depending upon the character of distribution desired.

« PreviousContinue »