356 PHYSIOLOGICAL PHYSICS. [Chap, xxvin. 



focus of a concave mirror, so that a greater amount of 

 light is thereby thrown 011 the lens for concentrating 

 on the plane mirror. By similar dispositions of 

 mirrors and lenses, other canals and cavities of the 

 body have been explored. 



MICROSCOPES. 



The simple microscope. It has been seen 

 (page 318) that when an object is placed between a 

 double convex lens and its principal focus, the cones 

 of light proceeding from various points of the object 

 do not meet after passing through the lens, but are 

 still divergent. No conjugate focus on the opposite 

 side of the lens is formed, but, instead, the pro- 

 longations backwards of the divergent pencils meet in 

 points on the same side of the lens as the object, but 

 outside of the principal focus. A virtual image is 

 thus formed, which, on account of its position, is 

 erect and highly magnified. (See Fig. 146.) It is thus 

 evident that a simple biconvex lens affords an easy 

 means of magnifying small objects, and rendering 

 them more clearly visible. The eye looks through 

 the biconvex lens, which has a small object on its 

 other side, nearer than its principal focus, receives 

 the divergent rays, focusses them on to the retina by 

 its own refractive media, and the image so produced 

 is referred outwards in the direction of the rays 

 falling on the retina, and the eye thus perceives the 

 highly magnified virtual image. It will be seen also,, 

 from reference to page 318, that the nearer the object- 

 is to the principal focus, while within it, the more 

 highly magnified is the object, and the nearer the 

 object is to the lens than it is to the principal 

 focus, the less highly magnified will be the image. 

 It is equally evident that the more convex the lens, 

 the more will the rays passing through it be refracted, 

 and when they do not converge, the more wide will 



