Criticism of Theories of Microscopic Vision. 159 



the eyepiece of the microscope and looking down the tube) 

 sends light to the eye in the same state as if that image were 

 a luminous surface, into the several points of which are con- 

 centrated the light of the above-mentioned plane-wavelet 

 components, in such manner that each point of image x 

 presents to the eye the same appearance as would the 

 concentrated light of one of these plane-wavelet components. 

 Since a point of image ,r transmits to the eye only an 

 infinitesimal quantity of light, it is too faint to be seen. 

 It requires a small extent of surface— what I have called 

 a macula — of this image to furnish light enough to be seeu. 

 Xow the smallest visible macula of image x will contain an 

 infinite number of points, and therefore presents to the eye 

 the appearance of the accumulated light of a whole sheaf, an 

 infinite number, of the trains of wavelets. This example 

 enables us to see that there is a physical relationship between 

 my theorem and the above-mentioned geometrical axiom. 



On p. 487 Mr. Wright supposes that the image of a self- 

 luminous object " can be only analysed according to the 

 Airy method/'' i. e. by an analysis of the light from the 

 object into spherical waves emitted from its several puncta. 

 From what is explained in the preceding paragraphs, it will 

 be anticipated that Mr. Wright is here under an entire mis- 

 apprehension. What happens in the case of the puncta, 

 or so-called points, of a self-luminous body, is the following. 

 Each punctum is a source of energy supplied through it to 

 the aether, and is surrounded by a small region of turmoil, i. e. 

 of local disturbance, in the aether *, which turmoil returns some 

 of its energy inwards in the form of a reaction against the 

 punctum, and expends the rest of its energy outwards in 

 starting spherical waves, which thenceforward advance for- 

 wards under the exclusive control of the forces inherent 

 in the medium. The character and size of the turmoil have 

 been studied by Hertz and others. It is not sensible beyond 

 a very few wave-lengths from the punctum. The rest of the 

 aether, beyond this small volume, is in a state of undulation ; 



* A turmoil of some kind must exist wherever there is any mutual 

 action "between the aether and the non-aether. By aether is to be under- 

 stood the luminiferous aether, an aether which has a definite texture ; and 

 by turmoil is to be understood, not disorderly motion, but a motion located 

 in one situation, and which does not advance in the way waves do. 



The most remarkable instance in which this turmoil, or locally situated 

 motion, in the aether is brought prominently to the notice of microscopists 

 is in the phenomenon which is called " optical contact,'' which is due to 

 " Stokes's layer,'' a special kind of turmoil, or local motion, which exists 

 in the aether close to the boundary between a rare and dense medium. 

 (Sep Phil. Mag. for October 1896. p. 348, and for December 1896, p. 524.) 



