10 PROPERTIES OF LENSES. 



A virtual erect and magnified image is seen by the 

 eye if the object is situated within the principal focus 

 of the lens, because the crystaline lens of the ob- 

 server C Fig. 7 has the power of bringing the rays 

 to a focus and forming a real image on the retina R, 

 on this principle the magnified image in the simple 

 microscope depends. 



Concave lenses can only formvirtual images no 

 matter what the distance of theobject. (Fig. 4). 



Up to the present the passage of light through 

 lenses has been considered in its simplest form, it 

 remains now to investigate the imperfections in the 

 resulting images, their causes, and the various 

 methods of correcting them. 



When a ray of light passes from one substance 

 through another a part is reflected at the incident 

 surface, a second absorbed in its passage, and a third 

 transmitted. The greater the thickness and number 

 of the transparent media through which the light is 

 transmitted, the greater is the quantity reflected and 

 absorbed. Since Microscopic Objectives are com- 

 posed of several lenses the loss of light is conse- 

 quently very great. 



Spherical Aberration shows itself in want of dis- 

 tinctness in an image at the margins when sharp at 

 centre and vice versa, it is caused by the unequal re- 

 fraction of the rays owing to the spherical shape of 

 the lens, those at the margins coming to a focus 

 before those at the centre. 



Chromatic Aberration ; a ray of white light is 

 composed of rays of different colours, if it pass 



