Virtual, magnified, and upright images
Opera or Field Glasses. - The opera glass consists of an objective LM which converges the rays toward the point S (Figure 43), but before they reach this point they pass through the diverging lens NP which replaces the eyepiece of the telescope. In passing through this diverging lens, the rays that were converging on entering it are made to diverge on leaving it. To an eye on the right-hand side of the lens NP the rays seem to have come from a point A' behind the concave lens NP, which thus forms the virtual and erect image A'B' of the object from which light was received. Unlike the astronomical telescope, the opera glass gives an erect image. To have the opera glass in focus, the lens NP must be so placed with respect to the objective that the rays emerging from the lens NP are nearly parallel.
Figure 43 - Opera or field glasses. They produce
Virtual and upright images
Standards of Illumination. - The intensity of illumination is measured by the amount of light which falls on unit area of a surface. The amount of energy received by unit surface cannot be easily determined in absolute measure. The eye is the most sensitive means of detecting light, but it does not give a quantitative measure of it. By means of the eye it is possible, however, to make an accurate comparison of two intensities of illumination.
To compare sources of illumination, a standard source of illumination is necessary. The choice of these standards is more or less arbitrary. There are a number of such standards in use. The British standard candle is defined to be a candle made of spermaceti, weighing six to the pound and burning at the rate of 120 grains per hr. This standard does not have a sufficiently constant illuminating power to make it of scientific value. Its illuminating power changes with atmospheric conditions and with the conditions under which it is burned.
Candle Power. - The candle power of a lamp is a specification of its illuminating power in terms of some standard candle. For example, the illuminating power of an incandescent lamp may be thirty times that of a standard candle and is then said to be a 30-cp lamp. But the illuminating power of a light varies according to the direction from which it is observed. It becomes necessary, therefore, to measure the average candle power in a given plane. The average illuminating power in the horizontal plane is called the mean horizontal candle power. The mean spherical candle power denotes the average illumination by a source of light from all directions in space.
The Foot-candle. - A Foot-candle is the intensity of illumination upon a surface at a point which is 1 ft distant from a source of 1 candle, the surface being perpendicular to the light rays at that point.
The Lumen. - The unit used to denote quantity or amount of light is the lumen. A lumen is the amount of light falling on a surface that has an area of 1 sq ft when every point of the surface is 1 ft from a point source of light of 1 candle.