OPTICS. 



which is painted with a different shade 

 of blue, decreasing gradually from the 

 deepest blue, formed by a mixture of 

 black, to the lightest, formed by a mixture 

 of white. This coloured zone is held in 

 the hand of the observer, who notices 

 the particular tint which corresponds to 

 the colour of the sky. The number of 

 this tint, reckoned from the greatest, 

 is the intensity at the time of observation. 

 Saussure, Humboldt, Depons, and other 

 travellers, have made observations with 

 this instrument. The following are 

 some of their results : 



General intensity in Europe 14 



General intensity in the Caraccas ... 18 

 General intensity at Cumana 24 



6. Coloured Shadows. The shadows 

 of bodies placed only in one light, and 

 at a distance from all other bodies ca- 

 pable of reflecting light, must necessa- 

 rily be black. In a summer morning 

 or evening, however, the shadows of 

 bodies formed either by the light of the 

 sun, or by that of a candle, have been 

 observed to be blue; this obviously 

 arises from the shadows being illumi- 

 nated with the light of the blue sky. 

 The colours thus produced vary in dif- 

 ferent countries, and at different seasons 

 of the year, from a pale blue to a violet 

 black ; and when there are yellow va- 

 pours in the horizon, or yellow light 

 reflected from the lower part of the sky, 

 either at sunrise or sunset, the shadows 

 have a tinge of green arising from the 

 union of these accidental rays with the 

 blue tint of the shadow. 



If the light of the sun, or of the 

 candle, be faint, then the shadow of the 

 body formed by the light of the sky will 

 be visible also, and the two shades will 

 be the one blue and the other a pale 

 yellow, two colours which are comple- 

 mentary to each other. This fact has 

 been ascribed to the circumstance of 

 the light of the candle, and that of the 

 rising and setting sun, being of a yel- 

 lowish tinge ; but though this will in- 

 crease the effect it is not the main cause 

 of it, as one of the shadows would be 

 yellow, even if the light 01 the sun and 

 the candle had been perfectly white. 



The phenomena of coloured shadows 

 are sometimes finely seen in the interior 

 of a room ; the source of one of the 

 colours being sometimes the blue sky,and 

 the other the green window blinds, the 

 painted walls, or the coloured furnimre. 



The best method of observing and 

 studying this class of phenomena is to 



use two candles, and to hold before one 

 of them apiece of coloured glass, taking 

 care to remove to a greater "distance the 

 candle before which the coloured glass 

 is not placed, in order to equalize the 

 darkness of the two shadows. If we 

 use a piece of green glass, one of the 

 shadows will be green, and the other a 

 fine red; if we use blue glass, one of 

 the shadows will be blue, and the other 

 a pale yellow, and so on ; the one colour 

 being always complementary to the 

 other, as explained in page' 46. The 

 light from the candle with the green 

 glass obviously illuminates the shadow 

 formed by the other candle, and hence 

 it is easy to understand why that sha- 

 dow is green ; but as the other shadow 

 is illuminated only by the common 

 light of the candle which is not red, it 

 appears difficult to discover the origin 

 of the red light. The explanation of 

 this must be sought not among optical, 

 but among physiological principles. We 

 have already seen, when treating of acci- 

 dental colours, that when a portion of 

 the retina was strongly impressed with 

 any one colour, such as red, that same 

 portion tinged green the images of white 

 objects that fell upon it. In like man- 

 ner, when nearly the whole retina is 

 impressed with any one colour, such as 

 red, a portion of it not impressed with 

 that colour will tinge white objects 

 green, or, to speak more generally, 

 every excitation of the retina by one 

 colour is accompanied by an excitation 

 of its accidental colour, just as in 

 Acoustics every fundamental sound is 

 actually accompanied by its harmonic 

 sound. Hence, when we see red we 

 at the same time see green, but its im- 

 pression is less forcible, and the ten- 

 dency of this double vision of colours is 

 to weaken the original impression, viz. 

 the red ; because the union of comple- 

 mentary colours produces whiteness. 

 This may be proved by looking for a 

 considerable time at a red water, which 

 will appear less and less red the longer 

 we view it ; because the green which 

 the retina is seeing at the^same time, 

 produces a whiteness which dilutes the 

 red. This we conceive to be the true 

 theory of accidental colours. Its appli- 

 cation to coloured shadows is very 

 obvious : When the eye is impressed 

 with the green colour of the light 

 transmitted through the green glass, it 

 at the same time sees red, which, of 

 course, appears only on the shadow 

 upon which a green light falls. 



