.^Vt':J^^tT^'|fo Cau^e of tolmired concenfnc i^/w^^r- ' ^^^'- S^ 



tcentre which T httvti iifever seen in any prismatic spectrum. 

 It is a kind of light brown, resembling the colour of a cer- 

 taifi'sort of Spanish snuff. The 170 feet object glass showed 

 the same colour also very clearly. 



XIII. Of the Order of the Colours. 



The arrangement of the colours in each compound ring or 

 alternation, seen by reflection, is, that the most refrangible 

 rays are nearest the centre ; and the same order takes place 

 when seen by transmission. We have already shown that 

 when a full dilution of the colours was obtained their arrange- 

 ment was violet, iridigo, blue, green, yellow, orange and red; 

 and the same order will hold good when the colours are 

 gradually concentrated again ; for though some of them 

 should vanish before others, those that remain will always 

 be found to agree with the same arrangement. 



If the rings should chance to be red and green alternately, 

 a doubt might arise which of them is nearest the centre; 

 but by the method of. dilution, a little pressure, or some 

 small increase of the focal length of the incumbent lens, 

 there will be introduced an orange tint between them, which 

 will immediately ascertain the order of the colours. 



In the second set of rings the same order is still preserved 

 as in the first j and the same arrangement takes place in the 

 third set as well as in the fourth. In all of them the most 

 refrangible rays produce the smallest rings. 

 XIV. Of the alternate Colour and Size of the Rings belong' 

 hig to the primary and dependent Sets, 



When two sets of rings are seen at once, and the colour of 

 the centre of the primary set is black, that of the secondary 

 will be white; if the former is white, the latter will be black. 

 The same alternation will take place if the colour of the centre 

 of the primary set should be red or orange ; for then the 

 centre of the secondary one will be green ; or if the former 

 happens to be green, the latter will be red or orange. At the 

 same time there will be a similar alternation in the size of 

 ringy ; for the white rings in one set will be of the diameter 

 of the black in the other ; or the orange rings of the forniA 

 will be of equal magnitude with the green of the latter. 



When three sets of rings are to be seen, the second and 



third 



