Limit to Defining-jpower. By J. J. Lister. 43 



viz. that the intrinsic defining -poiorr of pencils of light from an 

 object infected at circular apertures * varies in the ratio of the aper- 

 tures. The law is thus traced through a range varying from 1 to 

 5000 in diameter, and I see no reason to doubt its extending 

 indefinitely further. 



The effect is evidently the result of the dispersion and inter- 

 ference of the pencils of light proceeding from every point of an 

 object that go to form the picture; and their diffusion is well 

 illustrated by the round spots bounded by a dark circle which 

 have been mentioned as the images of bright points of insensible 



Aperture of the 

 Object Glass (a) 



Separation of Middles 



of Stripes'or Squares 



(S) 



1-5 in. 



5 ft. DoUond 



1-95 in. 

 2-65 in. 



2-7 in. 



3^ ft. TuUey, 



fine 



3-24 in. 

 5 ft. Dollond 



3*8 in. whole 

 aperture 



Checquer '0715 

 (square 'OS) 



Reflection -075 



of checquer 

 Stripes .. -058 



Stripes . . • 



Reflection • 

 of checquer 



Ditto 



058 

 0373 



•0373 



Reflection -0306 

 of stripes 



Distauce of Object from 

 Object Glass (d) 



pattern in. 



certainly mottled at 4560 



not visible at.. .. 4800 



(edge notched) 

 fairly seen at . . . . 4800 



canibe discerned at 3840 

 not visible at. . . . 3960 

 visible but woolly at 4800 



faintly mottled at 4380 



just vanishing at . . 4500 

 suspected at . . . . 4800 

 suspected at . . . . 4740 



visible at .. .. 4500 



invisible at 



4800 



d 

 S 



63776 



67020 



64000 



66207 

 68276 

 82758 



120693 

 128686 

 127077 



147059 



156862 



O H 



d 



■0000234 



■0000223 



•0000234 



•0000226 

 •0000219 

 •0000235 



■0000219 

 ■0000206 

 ■0000213 



■0000220 

 •0000206 



All the objects were too large to vanish 



magnitude viewed through small apertures. If sufficiently brilli- 

 ant, these are surrounded by prismatic rings with dark spaces 

 between them which may be observed to advantage by a good 

 telescope or achromatic Microscope adjusted to its correct focus with 

 a contracted aperture. The rings have each the blue side inwards 

 and the red outwards, the central spot being white, bounded by 

 dull red or brown. But upon gradually carrying the object out 

 of focus either way, the centre becomes altered in succession to 

 orange, red, dark purple, blue, greenish white, yellowish white, 

 and attain red, etc. ; those colours which last occupied the centre 



* The distance of the eye from the small circular apertures through which, in 

 the table just given, pencils of nearly parallel rays were admitted, was so short 

 that they may be practicaUy considered as if touchmg its surface. 



