1012 



THE SENSES 



parallel to the principal axis. Produce DI backwards (dotted line); 

 it will pass through the principal focus F. Through A draw the second- 

 ary axis AC. The image of A must lie both on AC and on IDF i.e., 



it must be the intersection, a. of these straight lines. Similarly the 

 image of B is b, the intersection of KGF and BC. The image is virtual 

 and erect. 



Absorption. No substance is perfectly transparent; in addition to 

 what is reflected, some light is always absorbed. In other words, in 



Fig. 413. Refraction by 

 a Biconcave Lens. 



Fig. 414. Formation of Image by Biconcave Lens. 



passing through a body some of the light is transformed into heat, a 

 portion of the energy of the short, luminous waves going to increase the 

 vibrations of the molecules of the medium, just as a wave passing under 

 a row of barges or fishing-boats set them swinging and pitching, and so 

 imparts to them a certain amount of energy, which is ultimately changed 

 into heat by friction against the water, and against each other, and bv 



the straining and rubbing of 

 the chains at their points of 

 attachment. Some bodies ab- 

 sorb all the rays in the pro- 

 portion in which they occur in 

 white light ; whether looked at 

 or looked through, they appear 

 colourless or white. Other 

 substances absorb certain rays 

 by preference, and the amount 

 of absorption is proportional 

 to the thickness of the layer. 

 The colours of most natural 

 bodies are due to this selective 

 absorption. Even when looked 

 at in reflected light, they are 

 seen by rays that have pene- 

 trated a certain way into the 

 substance and have then been 

 Fig. 4I5 . Diagram to show Connection of Body reflected ; and, of course, a 

 Colour with Selective Atisorption. smaller number of the rays 



which the body specially ab- 

 sorbs are reflected than of the rays which it readily transmits, for more 

 of the latter than of the former reach any given depth. This is called 

 ' body colour '; and such substances have the same colour when seen by 

 reflected and by transmitted light. The colour of haemoglobin is due 

 to the absorption of the violet and many of the yellow and green rays, 

 as is shown by the position of the absorption bands in its spectrum (p. 51 ). 

 In Fig. 4 1 5 the violet rays are represented as being totally absorbed before 

 passing through the substance. Some of the green rays are reflected, some 



