Limit to Defining -poirer. Bi/ J. J. Lister. ~)b 



though it may not admit of such positive mecasurement as the 

 separation of squares or stripes furnishes, requires to l)e adverted 

 to, namely, that whicli regards distinctness of outline and the 

 extreme limit to the visibility of a single object. 



The smallest angle under winch I could detect the existence 

 of a black square on a wliite ground with the unassisted eve in 

 daylight was w^hen its side subtended 33 seconds, which nearly 

 agrees with Sir W. Herschel's and Mayer's observations, and is 

 about half the angle at which separation vanislies in a series of 

 stripes or a chequer (p. 9) ; but viewed through circular apertures 

 of 0'02 in. and under, and in the Microscope, it is just visible as 



f f 1 f fi f 1 0-000007 T , . ., , 



far as to ^ oi that angle, or , radius being unity. An 



equal white square on a very black ground may be detected at 

 more than double the distance of a black on wliite, though its 

 limit has been stated to be the same. Indeed, the angle under 

 which a bright object may be seen diminishes indefinitely with 

 the increase of its intensity and its opposition to the space sur- 

 roundinor it. 



Merely to ascertain the existence of a something is, however, 

 generally of little value unless we can also distinguish its figure, 

 and here our vision is confined to much narrower limits. Sir W. 

 Hersehel determined that a square area could not be distinguished 

 from an equal circular one with the naked eye till the latter sub- 

 tended 2' 1 7", and I found that it must be still larger to have its 

 form well defined. The black and white squares composing a 

 chequer could, when view^ed through small circular apertures or 

 in the Microscope, be seen to be of the same size and to touch at 

 their corners till brought to w^ithin half the vanishing distance of 

 the pattern, i.e. to a distance when the side of a square is at an 



. , . 0-00005 

 angle oi about . 



(a/ 



In various trials made by me, the light beyond the green, 

 towards the violet end of the spectrum, was found too faint to 

 give that increase of definition that might be expected from it, 

 except when the solar beam was thrown on the prism, and although 

 the blue rays then produced an eftect of singular beauty, they 

 were, like the sun's light in general, so accompanied with fring:es 

 as to prevent their showing objects in the Microscope with dis- 

 tinctness and certainty. 



