PHYSICAL PROPERTIES OF LIGHT 489 



completely all colours which belong to the other end of the spectrum. (Basic acetate 

 of copper, i.e. verdigris, has this property.) Lastly the case of a fluid may be given which 

 absorbs colours in the middle of the spectrum, while it transmits freely those at the 

 ends. The light transmitted is therefore violet in colour, (as the appearance of a solution 

 of potassium permanganate or methyl violet shows.) Colour is thus due in every case 

 to some difference in the behaviour of the substance towards the various rays of the 

 spectrum. In order to complete the description of colour formation, two other methods 

 should be described by which colour may be produced, namely fluorescence and phos- 

 phorescence. The former term is applied when a substance absorbs light of one colour, 

 and at the same time emits light of another. The latter is applied when a substance 

 emits light for an appreciable time after the exciting light stimulus has ceased. 



LIGHT SOURCES fall into two classes, those which emit radiant energy because 

 of the high temperature to which they have been raised, and those which are excited in 

 other ways. The light from the former class as a rule consists of rays of all wavelengths, 

 from the longest heat rays to the shortest ultra-violet. The light from the latter 

 class on the other hand, is frequently found to consist of rays corresponding to char- 

 acteristic regions of the spectrum. The mercury vapour lamp may be mentioned as 

 an example of the latter type of light source, which has come into general use. If 

 the visible spectra obtained from a few light sources of the first type are carefully 

 measured, it is found that although rays of all wavelength are present, there are con- 

 siderable variations in the intensities of the different rays. This causes variation not 

 only in the colour of the light as a whole, but also affects the colour of objects and the 

 ease with which the eye can judge colours. This variation in the distribution of intensity 

 in the spectrum is found to be accompanied by corresponding changes in the infra- 

 red and ultra-violet. For equal visual intensity, as the temperature of the source is 

 increased, the greater is the amount of ultra-violet light and the less is the infra-red. 

 Moreover as the temperature is raised, the whiter, and therefore the more like daylight, 

 does the light become. Measurements of the energy present in different parts of the 



10000 



HEAT VISUAL ACTINIC 



FIG. 244. Curves shewing relative energy and luminosity of different regions of 



the spectrum. 



spectrum show that much the greater part of the energy is present in the infra-red. 

 These heat rays play no useful part hi vision, and may in fact do harm ; the greater 

 part of the energy is thus wasted, and for this reason the efficiency of this class of 

 light source is very low. Since raising the temperature of the light source causes the 

 light to approximate more closely to daylight and at the same time reduces the 

 relative amount of the infra-red rays, it effects considerable advantage because it 

 increases efficiency. 



THE ENERGY IN THE SPECTRUM is present in greatest amount at the red 

 end, and least at the violet. In spite of this the part of the spectrum with the greatest 

 luminosity to the eye is the yellow. The values obtained by Abney are shown in 

 figure 244. 



DIFFRACTION AND REFRACTION. Beside the properties of light that have 



