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From thefe few principles we may determine the 
phenomena of light tranfmitted through parallelepi- 
pids that are contiguous to the air, their pofition and 
refractive powers being given. Or we may difpofe 
them fo that the emergent light fhall, or fhall not, be 
tinged with colours. 
And we already fee (what (hall be more diftin&ly 
explained below) that if light be tranfmitted through 
whatever number of media (A, B, C, &c.) all the re- 
fractions may be corrected by the equal and contrary 
refraCtions of the fame number of the fame media 
7,) fimilar and limilarly lituated to the former; 
provided there is a medium Z interpofed between 
the two feries, thus; A, B, C, Z, c,h,a ; and that 
the rays in their palfage through Z, are parallel to 
one another. 
7. But to give the rays this parallelifm in their paf- 
fage through Z, and to explain the feveral phenome- 
na of refraCted light, we fhall need the following 
LEMMA, a Problem. 
Given (in Fig. 2.) DCB the difference of two an- 
gles A CD, ACB, and the ratio of DI the fine of 
the greater to B H the line of the lelfer being like- 
wile given, to find the angles. 
For DF, the line of the given difference, write s , 
and for its coline C F write c ; for the lelfer line B H, 
the letter z, and let the given ratio of D I to B H, be 
that of m to », the radius C B being unity. 
Then, having drawn F G perpendicular to DI; 
from the fimilar triangles in this figure, we (hall have 
C B 
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