THE YOUNG NATURALIST. 18 



E A 



instead of passing straight on to C it is bent 

 to D. This ray will be bent more or less 

 according to , the density of the material it 

 enters. But all the light does not pass 

 through the water, but a portion of it is 

 refracted or thrown back to E, and these reflec- 

 ted rays always leave the surface of the water 

 at the same angle at which they touch it. 

 It sometimes happens that all the rays are 

 reflected, as you may see, by holding a 

 tumbler of water in a ray of light at a cer- 

 tain angle, when all rays will be reflected 

 upwards, while under the glass it is quite 

 dark. Passing by this general law, we next 

 come to a lens. It may be taken for granted 

 that the curved surface of a lens is some 

 part of a circle. This diagram represents 

 what is known as a plano-convex lens ; that 

 is, plain or flat on one side, and convex on 

 the other. 



Now, if we cause parallel rays of light to 

 fall upon the curved side of the lens, those 

 rays are brought to a focus, at the opposite 

 side of the circle of which the curved sur- 

 face forms a part. That is, the outside rays 

 of light falling upon the glass obliquely are 

 bent, and are again bent in the same manner 

 upon leaving the glass. The action of para- 

 llel rays, upon a double convex lens would be 

 to bring them to a focus, not at the opposite 

 side af the circle as before, but in the centre, 



because the rays reach the inner'surface of 

 the lens at a greater angle, and consequently 

 come to a focus sooner. If the rays diverge 

 before they reach the lens, they are brought 

 to a focus at a greater distance from the 

 lens. If they be converging rays which fall 

 upon the lens, they are brought to a focus 

 sooner, or before they reach the centre. A 



lens will make an image as you may see by 

 holding a lens at a proper focus between a 

 lighted candle and a sheet. It is upon these 

 principles that all our microscopes, both 

 simple and compound, are constructed. 

 But when they begun to make microscopes 

 they found several difficulties were in the 

 way ; two particularly. One of these diffi- 

 culties was what is called "spherical aber- 

 ration." I will try and explain the meaning 

 of this. The rays of light which fall upon the 

 outside of the lens come to a focus before 

 those which pass through nearer the centre, 

 consequently all the field or all the object 

 under the microscope could not be brought 

 into focus at the same time. The easiest 

 way and the way mostly employed now in 

 making cheap microscopes is to put in wliat is 

 called a stop, that is a circular piece of opaque 

 material with a hole in the centre, so placed 

 as to prevent the outside rays from passing. 

 It is then only necessary to make the lens 

 larger than required and use the central 



