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 diftinct 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 diftinctly, 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. 



If 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 iky, 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 corrected, 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 fufficiently difperiive 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 fpectrum, would be the fame as in them. Such a 

 colourlefs refraction 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. 



If 



