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A MANUAL OF PHYSIOLOGY 



FIG. 284. FORMATION OF IMAGE BY BICONCAVE 

 LENS. 



Formation of Image by Biconcave Lens (Fig. 284). Let AB be 

 the object. Let AHDI be the path of a ray from any point A of 



the object parallel to 

 the principal axis. 

 Produce DI back- 

 wards (dotted line) ; 

 it will pass through 

 the principal focus F. 

 Through A draw the 

 secondary axis AC. 

 The image of A must 

 lie both on AC and 

 on IDF; i.e., it must 

 be the intersection, #, of these straight lines. Similarly, the image 

 of B is , 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 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 sets them swinging and 

 pitching, and so imparts to them a 

 certain amount of energy, which is ulti- 

 mately changed into heat by friction 

 against the water, and against each other, 

 and by the straining and rubbing of the 

 chains at their points of attachment. 

 Seme 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 absorp- 

 tion bands in its spectrum (p. 48). In Fig. 285 the violet rays are repre- 



FIG. 285. DIAGRAM TO 

 SHOW CONNECTION OF 

 BODY COLOUR WITH 

 SELECTIVE ABSORPTION. 



