S9 6 A MANUAL OF PHYSIOLOGY 



but if the focus is to be sharp, the angle between the secondary and 

 the principal axis must not be so large as is indicated in Fig. 376. 



Formation of Image by Biconvex Lens (Fig. 377). Let AB be the 

 object ; then if AHD be the path of a ray from A parallel to the 

 principal axis, the image of A will be the intersection of the straight 

 line DF and the secondary axis passing through A. Similarly, the 

 image of B will be the intersection of GF and the secondary axis BC. 

 Where AB is farther from the lens than the principal focus, the image 

 ab is real and inverted. This is the case with the image of an 

 external object formed on the retina. When the object is nearer 

 than the principal focus, the image is virtual and erect. The image 

 formed by the objective of a microscope when the object is in focus 

 is real and inverted ; the ocular forms a virtual erect image of this 

 real image. 



Refraction by a Biconcave Lens (Fig. 378). Parallel rays are 

 rendered divergent by the lens ; there is no real focus ; but if the rays 

 are prolonged backwards they meet in the virtual focus F, from 

 which they appear to come when received by the eye through the 

 lens. 



Formation of Image by Biconcave Lens (Fig. 379). Let AB be 

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



FIG. 378. REFRACTION FIG. 379. FORMATION OF IMAGE BY BICONCAVE 

 BY A BICONCAVE LENS. LENS. 



the object parallel to the principal axis. Produce DI backwards 

 (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, 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 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 ultimately 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. 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. 



