APPENDIX. 



A short note concerning the influence of diffraction on the resol 



power of microscopical ohjectives, and on the apparent colour of 

 microscopical objects. 



By Dr. G. Johnstone Stoney, P.E.S. 



If we look from a distance at a flame through a thin feather 

 or other uniformly ruled grating we see the flame, and 

 around or on either side of it a number of lateral coloured 

 images, which are wider and usually fainter the farther out 

 that they lie. We thus learn that the light which passes 

 through the grating becomes both a direct beam and a 

 number of lateral, or diffracted beams as they are called. 

 The proportions in which the light which passes the 

 grating is distributed between the central beam and the 

 several diffracted beams depends upon the ratio of the 

 widths of the openings to the widths of the bars of the 

 grating, as well as upon such particulars as whether each 

 opening is a mere hole and each bar a mere obstruction, or 

 whether they are occupied by material which acts on the 

 light, especially if it act like a prism. It rarely happens 

 that this distribution does not perceptibly differ for light of 

 different wave-lengths. The direct beam consists of light 

 in very nearly the same state as if it had passed through a 

 simple opening of the size of the grating, except that it is 

 fainter — usually fainter in some colours than in others. 



Accordingly, if the eye when looking at the grating, or 

 if the object-lens of a telescope, were to receive only this 



