53 
crystallized bodies on homogeneous light. 
Sulphate of soda ? Arragonite, 
Sulphate of baryta, Sugar, 
Nitrate of potash, Hyposulphite of strontia, 
it will be seen that the tints between the poles PP' correspond 
to lower orders of colour, than would result from assuming 
P, F, as the origin of the scale, and agree much better with 
the assumption of certain joints p,p' without the poles, as their 
zero, or commencement of the scale. The poles themselves, 
too, instead of being absolutely black, are tinged with colour; 
and the tints beyond them, instead of descending in the scale 
from the poles outwards, continue to rise till they reach their 
maximum (which is a white, more or less brilliant, or an abso- 
lute black) at the points p,p ' ; after which they descend again 
to infinity. Not that in any case they coincide precisely with 
the scale of Newton, even with this correction, but, except 
in extreme cases, approximate to it within some moderate 
limit of error. 
If, on the other hand, we examine in the same manner one 
of the following bodies : 
Tartrate of soda and potash, Sulphate of magnesia, 
Borax, Topaz, 
Mica, 
it will be found that the imaginary points p,p' ( which we shall 
call the virtual poles ) from which the tints must be reckoned 
inwards and outwards, to produce the nearest possible agree- 
ment with Newton's scale, lie between the poles P, P'. 
In all these crystals, as the thickness of the plate examined 
increases, the virtual poles pp' recede from the actual ones 
PF, at least in respect of the number of alternations of colour 
which intervene between them : in other words, the tint deve- 
