68 THE MICROSCOPE. 



is a common cause of annoyance, as well as a source of 

 embarrassment, to the microscopist, the disturbance pro- 

 duced by the passage of light through transparent bodies ; 

 this greatly impairs the sharpness of outline, and is caused 

 by what is called the inflection or diffraction of light. 



If a divergent pencil of light fall upon any opaque 

 object placed between the light and a screen, it is evident 

 that a shadow will be cast upon the screen : from our 

 previous observation that light always moves in straight 

 lines, we should expect to find that the image would be 

 clear, sharp, and well-defined ; but we know from experi- 

 ment that this is not so, and that there will be no exact 

 margin between the strongly illuminated part outside, and 

 that part of the screen which is covered by the object, but 

 there will be more or less of a shading off at the edges of 

 the image. 



From this it is inferred that the rays of light which 

 pass the edges of the opaque object do not proceed in the 

 same straight lines in which they would have proceeded if 

 the object had not been present. This effect is called in- 

 flection or diffraction, and is a consequence of the general 

 property of undulation. When any system of waves meets 

 with an obstacle, subsidiary systems of undulation will be 

 formed round the extremities of the obstacle, and will be 

 propagated from those points independently of, and simul- 

 taneously with, the original system of waves. And for a 

 certain space around the lines in which the rays, grazing 

 the edge of the opaque body, would have proceeded, the 

 two systems of undulation will intersect and produce the 

 phenomena of interference. 



If the opaque body be very small, and the distance 

 from the luminous point very large in proportion, the two 

 pencils formed by inflection will intersect, and all the 

 phenomena of interference will ensue. Thus, if the light 

 be homogeneous, a bright line of light will be formed 

 under the centre of the opaque object, outside of which 

 will be dark lines, and then bright and dark lines alter- 

 nately. If the light be compound solar light, a series of 

 coloured fringes will be formed. The following is a very 

 good illustration of how errors of interpretation may 

 easily arise. If a small sphere formed of any opaque sub- 



