B42 Hersclici on the DcmationsJ'rom Nexvto'iis Scale 
tains the value of which measures the polarizing energy of 
the crystal on that particular ray ; the last the number of obser- 
vations employed in computing the values in the preceding. 
Table III. Representing the Laze of Action of the 
Second Variety of Apophyllite^ on the differently 
coloured Rays of the Spectrum. 
Name of Colour. 
Value of h 
^ 1000000 
Value of ^ • 
Number of 
Observations. 
Extreme red. 
20213 
49.475 
20 
Mean orange. 
25465 
39.270 
20 
yellow. 
S0374 
32.923 
20 
green. 
38057 
26.277 
20 
i)lue. 
93904 
10.649 
10 
indigo. 
250000 -f 
4.000 — 
• — 
Indigo violet. 
250000 -f 
4.708 — 
— 
Mean violet. 
45992 
21.143 
2 
Extreme violet. 
35043 
28.536 
13 
By this Table we see that the action of the crystal decreases 
rapidly, but regularly enough, from the extreme red to the blue 
rays, when it sinks all on a sudden, and throughout the whole 
extent of the indigo and first portions of the violet is so small, 
that I was unable to obtain a measure even of the first ring at 
its maximum.^ within the range of incidence my apparatus would 
admit. It then increases again, more suddenly. than it fell, and 
for the extreme violet has a value intermediate between those 
for the yellow and green. If we construct a curve r oy gh iv, 
Plate VII. Fig. 14. whose abscissas AR, AO, &c. are recipro- 
cally proportional to the lengths of the Newtonian fits, and draw- 
ing the line Ar B at an angle of 45° with A V, take the ordinates 
R r, O o, &c. every where proportional to the value of 
in this Table, this curve will represent the action of this variety 
of apophyllite on the whole spectrum, while r B represents^that 
of a crystal whose tints follow the Scale of Newton. 
This is in perfect agreement with the succession of tints gi- 
ven above. If we cast our eye over it, a deficiency of the indigo 
rays is perceived in all the scale of the extraordinary pencil, no 
pure or rich blue occurring throughout its whole extent. In the 
ordinary pencil, on the other hand, the excess of indigo appears 
