36 Transactions of the Society. 



an object and also in the Microscope gave for the images of bright 

 points or minute spots the same spurious disks that are presented 

 by stars in the telescope, of which, indeed, the compound Micro- 

 scope is but a modification ; the character of these disks will be 

 again adverted to, but it is here observable that, although they 

 enlarged as the pencil was much contracted, their brightness, either 

 absolute or in contrast to the space surrounding them, prevented 

 their diminution with the larger apertures from being appreciated 

 till the light was modified. It was therefore necessary that such 

 objects and such an illumination should be selected for the in- 

 vestigation as might be best adapted to employ and to measure 

 the full defining power of the eye. 



After various essays of different kinds, patterns, of several sizes 

 of black and white squares and stripes, were fixed on. They were 

 made at first of black and white paper, but afterwards, to give the 

 greatest facilities for use, they were prepared by pasting upon 

 glass black paper, cutting it witli parallel lines at regular distances 

 by a machine and stripping the paper from the alternate spaces so 

 as to show light transmitted from behind. 



By crossing two series of stripes of the same size at right 

 angles, squares were obtained distant the length of a side from 

 each other, and at oblique angles, rhombs ; also by filling up each 

 alternate white stripe or stripping off each alternate black one, 

 the black on the white stripe was made three times thicker than 

 the other ; this gave a variety sufficient for my purpose. 



The patterns were found to be best seen by transmitting from 

 a mirror behind them the light of a white cloud on a sunny day, 

 and the measures of the distances at which their separation vanished 

 were chiefly taken with such light. When the whole pupil or the 

 larger apertures were used, this was sometimes required to be 

 softened by interposing thin gauze paper. The vanishing distance 

 was not reduced more than about 2V when the sky was much 

 overcast, but it diminished greatly as twilight came on. The 

 light of an Argand lamp thrown strongly on ground glass required 

 the pattern to be brought about yV; nearer than that of a bright 

 cloud to render it visible. 



The effect was tried of viewing the pattern through blue and 

 other coloured glasses. Of these a green glass, which intercepted 

 the red, the violet, and part of the orange rays, was the only one 

 not detrimental, and even this was with the larger apertures 

 slightly so ; but from -^^ inch downwards a small increase of 

 distinctness, amounting on an average to ^^ in daylight, was the 

 estimated result of several observations by 4 persons out of 5, the 

 fifth, though of acute sight, and not differing from the others in 

 the estimation of colours, uniformly finding an opposite effect. 

 Lamplight, on the other hand, was plainly improved by the green 

 glass, so as to differ only about Jq from that of a cloud. 



After a few observations it was found (as Fraunhofer had ob- 

 served before) that the vanishing distance of a pattern depended 



