88 Scientific Proceedings, Royal Dublin Society. 
two tenthet-millimetres. Here, for the present, our powers end: | 
and we cannot fail to be impressed by the extraordinary accuracy 
which has been attained in measuring wave-lengths by the 
methods spoken of above. It is a degree of accuracy which — 
ascertains the length of a wave of light within a millionth of 
its entire length, thus equalling and 
even surpassing the best results ob- ieee "s ae ae 
ROM THE Sun, in Merro- 
tained when comparing with excessive ane 
care international standard yards or (The Sub-section Bu provides 
metres; in which a determination with- Sor all of these.) 
in one fifthet (the 100,000th) of the 
: Group B. 
whole length is probably the most that 
| 
| Puanerary Inrervats. 
can be fully relied on. faa 
q 
alee 
Group B (Pranrrary INTERVALS). fe ane 
= s 2D 
3 = eee 
We have next to direct our atten- S| eis 
tion to Nature’s operations on a great B d ese acs eS 
scale, and first to Group B which deals AR eear eely!|.< 
with events within the solar system. 5 a s i 5 
i : i D D FI 
This group, like the others of our sur- Se el enie 
vey, may conveniently be divided into Buea 
sub-sections—w, v, and w. Olaneann 000 0 
Bu, the sub-section of large plane- une 
tary measures, indicates the place in | 1 | = One, Metro. 
our table in which to record the dis- ale 
tances of the planets from the Sun, or as 
: : :5°8 Mercury. 
from one another, as is seen from fig. 1. eS Wena 
These distances are most conveniently 15 Barth. 
read out as somany metro-tens. 22-8 Mars. 
The next sub-section, Vv, makes Here come the minor planets. 
similar provision for representing the (78) Jupiter. 
distances of the satellites from their 143) Saturn. 
primaries, and for recording the size of ae ee 
the Sun, which belongs to the same 2 spine 
order of magnitude. This appears Fie. 1. 
from fig. 2, in which the distances may conveniently be 
expressed as so many Harth-quadrants, meaning by the “quad- 
rant” 1000 stages, or 10,000 kilems, which is approximately 
