194 LECTURES TO SCIENCE TEACHERS. 



large an angle. The magnifying power of a single lens thus 

 depends on its "bending and making more or less parallel rays 

 that would otherwise enter the eye at a very divergent angle. 

 We may indeed obtain a similar apparent enlargement by 

 looking through a minute hole in a blackened card, which 

 cuts off the divergent rays, but in this case so much light is 

 lost, and the diffraction is so great, that the definition is bad. 

 We thus see that the so-called magnifying power of simple 

 lenses is due merely to the fact of their enabling us to see 

 objects distinctly when they are very near to the eye. 



Single lenses are useful for examining an object with com- 

 paratively low powers, but they do not give perfect definition 

 unless they are only very small segments of spheres and none 

 of the rays are very much bent. They do not bring all the 

 rays to an exact focus, but have what is called spherical 

 aberration. The rays of light passing near the centre are 

 brought to one focus, those passing further from the centre 

 are brought to a focus nearer to the lens, and those at the 

 extreme edges still nearer. The result is that in a single lens 

 made with spherical curves you never can have an object 

 exactly in focus, since you have different focal lengths for 

 different parts of the lens. That could be overcome by 

 lenses with elliptical or hyperbolic curves, which would bring 

 all the rays to one focus, but the difficulties of making them 

 would be so great that, practically speaking, they cannot be 

 made in a satisfactory manner. I draw your attention to 

 the fact because the so-called spherical aberration with which 

 we have to contend in making microscopes is a matter of 

 very great importance, and it is very desirable that the true 

 source of the difficulty should be known. 



Various combinations of lenses have been made in order to 

 avoid as much as possible this spherical aberration. It would 

 occupy too much time to enter into details, but by combining 

 different lenses the spherical aberrations may be so greatly 

 reduced that very valuable doublet and triplet lenses may be 

 constructed, which are exceedingly useful for so-called simple 

 microscopes that is to say, microscopes having only a single 

 system of lenses. The chief advantage of simple microscopes is, 

 that the object is seen in its proper position it is not inverted. 

 In dissecting and in preparing objects they are very useful, 

 because all the motions of the hand are seen in the natural 

 direction. We are, however, limited very much in their use 



