558 Notes. 



appendage rings, or else portions of two or more of these rings if 

 all light from the spurious disk is excluded. If the aperture of the 

 objective is cut down till the disk only is admitted, the definition 

 will have become so bad that the outline of L cannot be seen. The 

 most satisfactory image of L will be formed, when the spurious 

 disk and all its diffraction appendages that have any appreciable 

 brightness are admitted by the telescope objective. 



The degree of definition — i.e. the steps from good to poor 

 definition — can be easily investigated by the " flat wavelet resolu- 

 tion " analysis, and it also shows that when the source of light A is 

 enlarged, there is no necessity for any phase relation between the 

 portions of light emanating from different puncta of A. The need- 

 ful phase relation — the one necessarily subsisting — is that between 

 the disk and the diffraction appendages formed from each separate 

 punctum of A. It follows, and is confirmed by experiment, that 

 the source of light may, without loss of definition, be a self- 

 luminous body. 



To understand how Lord Eayleigh's experiment, and others 



related to it, can be made with the Microscope, remove lens L of 

 the apparatus represented by the figure in the text, and replace it 

 by two lenses L' and L", of which lens L' collimates the light from 

 A, while L" concentrates the collimated beam to a focus at C. It 

 is obviously legitimate to make this substitution. When the 

 experiment is made with a Microscope, the source A is to be light 

 passing through a small hole (or slit) in a stop placed under the 

 condenser. The condenser of the Microscope then takes up the 

 duties of lens L', and at the same time the objective of the Micro- 

 scope discharges the functions of the combination consisting of 

 lens L" together with lens C. The image of A produced at C then 

 becomes that image of the hole (or slit) which may be seen in the 

 " concentration image " of the Microscope — i.e. in the image which 

 comes into view on removing the eye-piece and looking down the 

 Microscope tube. Furthermore, when the experiment is made 

 with the Microscope, any desired object can be put upon the stage 

 of the Microscope, and becomes the object to be resolved. The 

 author considers the best object to employ is one of the bands of 

 Grayson's Eulings, supplemented by observations upon a single 

 pair of lines such as may here and there be seen to project from 

 one or other end of a band. The hole in the stop may, if desired,. 



