66 On the UNEQUAL 
rallel lines which denote four o’clock in the enamelled dial-plate 
of a watch, and make the watch approach the eye very flowly. 
So long as the eye can conform itfelf to the diftance, the black 
lines will appear diftinét and of their proper colours. But when 
the watch, continuing to approach, is brought too near for the 
eye, by any effort, to fee the lines diftin@lly, the coloured 
fringes will begin to make their appearance, and the fpreading 
of the lefs refrangible rays into the black ftrokes, and the more 
refrangible rays into the white intervals, will make them appear 
to change their colours from black and white to orange and 
blue. 
Ir any doubt fhould remain concerning the prifmatic colour 
produced by the refraction of the humours of the eye, let the 
obferver look at a bar of the window, where it is oppofed to 
the fky, and holding his hand parallel with the bar, bring it 
flowly over his eye, he will obferve, juft before the bar dif- 
appears, one fide of it edged with red inclining to orange, and 
the other with blue, and thefe colours in as great quantity as 
would be produced by a prifm of a pretty confiderable refract- 
ing angle. The application of thefe obfervations to what was 
before faid of the fringes of colour produced by fimple and 
compound lenfes, is obvious. If the aberration from difference 
of refrangibility were perfectly correéted, no colour whatever 
would appear, either in the penumbras, or on covering part of 
the pupil. Had this been effected, it is probable that the vi- 
treous humour would be found fufliciently difperfive to correct 
the colour produced by the aqueous and cryftalline humours, 
and that the ratio in which it feparated the rays which form the 
coloured fpeétrum, would be the fame as in them. Such a 
colourlefs refraétion might then be produced as has been found 
to arife from a combination of crown-glafs with a fluid me- 
dium, containing a due proportion of metallic particles and 
particles of marine acid, 
Ir 
