[ 499 ■] 



LI. Microscopic Vision. 

 By G-. Johnstone Stoney, M.A., D.Sc, F.R.S. 



[Continued from p. 442.] 



[Correction in Part I. — Instead of the first line of the 

 footnote on p. 336, read as follows: — 



If the number of replicas is not absolutely infinite, the luminous effects 

 produced in the definite directions spoken of in the text are each a 

 maximum of maxima, &c] 



Part III. 



Contents. 



Page 



Definitions 4U9 



33. Numerical Aperture, or Grasp 500 



34. Information supplied by Image x 503 



35. Resolution . . 507 



36. Significance of what is seen in the Microscope . . 512 



37. Explanation of why, when the focus is changed, 



bright specks may become dark, and fine detail 



may seem to shift upon an object 513 



38. Illustrations of these Phenomena 516 



39. Illustrations of Illusory Colouration 518 



40. Illustrations of Fictitious Markings 520 



41. Optical Contact 524 



42. How to See the Rulings 525 



43. Concluding Remarks , 527 



Definitions. — It is convenient to collect here for reference 

 the definitions of some terms we shall have to use. 



Image x is that image which is seen on removing the 

 eyepiece and looking down the body of the microscope. It 

 is seen without distortion if the iris diaphragm below the 

 condenser occupies its proper position. (See p. 435.) 



The X scale is one of the radii of image x divided into 

 a scale of equal parts, with zero at the centre, and on which 

 the number G (the NA of the objective) is at its outer end. 

 (See p. 506.) 



A punctum means a luminous point in image x. It is what 

 a beam of parallel light becomes after passing through the 

 objective. (See p. 503.) 



A macula is a spot of light of any size and shape in image 

 x, and corresponds to a sheaf of beams emitted from the 

 object. (See footnote on p. 510.) 



If kg, in the figure on next page, be the front of the 

 objective, and o the middle of image C, then the radiating 

 lines represent the axial rays of a fan of beams as they emanate 

 from image ; and the feet of the perpendiculars are in the 

 relative positions in which the puncta of those beams will 

 present themselves in image x. (See p. 504.) 



