432 Dr. G. J. Stoney on Microscopic Vision. 



where n*, 1, and n' are respectively the indices of refraction 

 of the medium in front of the objective, of air, and of the 

 vitreous humour in the eye of the observer. 



Hence the ruling to which these two beams give rise in 

 image C is reproduced in images D, E, and F. The same is 

 true of all the rulings by the co-existence of which image C 

 is formed ; and as within each image the spacings of all the 

 rulings are modified in the same way, it follows that every 

 feature which is present in any one of these images is present 

 in all the others — of course on the supposition that no defect 

 is introduced by imperfections in the objective, the eyepiece, 

 or the eye. 



Image E appears to the observer like an object presented 

 to his unassisted eye, and in it the conditions must be fulfilled 

 for naked-eye vision. These chiefly concern the angular size 

 which that image must have in order that the minutest detail 

 in it may be sufficiently large to be satisfactorily seen (see 

 Section I. of a paper " On the Limits of Vision," Scientific 

 Proceedings of the Royal Dublin Society of December 20, 

 1893, p. 228 ; or, Philosophical Magazine for March 1894, 

 p. 317). 



But as to what the detail is, and as to the true nature of 

 that detail, we still have to fall back upon a study of standard 

 image No. 2, and of the adjustments of the microscope which 

 are available for improving that image. And this can be best 

 done by the study of individual examples, which is to be the 

 subject matter of Part III. 



If any difficulty still hangs over the subject treated of in 

 the last few sections it will be cleared up by following out 

 the whole history of one individual beam. 



27. Course of an individual beam traced, and How to see 

 it. — It has already been proved that the microscopic object 

 and all the illuminating apparatus may be removed, and that 

 standard image No. 2 may be substituted for them, if this 

 image be formed after two reversals, and if it therefore emits 

 its light forwards. Let C in the figure be the position of this 

 standard image and let Qb be one of the beams of uniform 

 plane waves which it sends forward. Let the dark line 

 in the figure represent the axial ray, that is the ray which 

 starting from the middle of image C continues throughout 

 perpendicular to the waves of which the beam consists. The 



* It is the usual practice in England to employ the symbol jx for the 

 refractive index ; but it cannot conveniently be so used in treating of the 

 microscope, since /j. is wanted as the designation of the micron. On this 

 account the continental practice of using n for the refractive index has 

 been adopted in the text. 



