972 



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 

 wliat is reflected, some li^ht is always absorbed. In other words, in 



Fig. 394. Refraction by 

 a Biconcave Lens. 



Fig. 395.- 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 

 mto heat by friction against the water, and against each other, and by 

 the straining and rubbing of the chains at 

 their points of attachment. Some bodies 

 absorb all the rays in the proportion 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 penetrated a certain way into the 

 substance and have then been reflected; and, 

 of course, a smaller number of the rays which 

 the body specially absorbs 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. 396 the 

 violet rays are represented as being totally absorbed before passing 

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

 transmitted, some absorbed. The red rays are supposed to be mostly 

 reflected and transmitted, only to a slight extent absorbed. The colour 

 of such a substance, both when looked at and when looked through, 



Fig. 396. Diagram to show 

 Connection of Body Colour 

 with Selective Absorption. 



