mating to the Construction of the Heavens . 
%9S 
. 13. Of very narrow long Nebula. 
In order to advance in our knowledge of the condition of 
the nebulous matter, we may investigate the form of its ex- 
pansion by the figure of the nebula that have been observed. 
The following five are particular instances of some that were 
much extended in length, but very little in breadth.* 
No. 254 in the 3d class is “ A very faint nebula, extended 
fC from north-preceding to south-following. It is about f 
s< long and less than % minute broad. ” See fig. 7. 
• The expansion of the nebulous matter in general may be 
considered as consisting of three dimensions ; these may all be 
either nearly equal, or one of them may be much less than the 
other two ; or the extent of two of them may be very inferior 
to that of the third. The nebulas which have now been referred 
to exclude a nebulosity of three nearly equal dimensions, 
which can never be seen under less than two of them. When 
two of the dimensions of the nebulous matter are nearly equal ? 
one of them may indeed be only visible ; but then the chance 
that the other should be exactly parallel to the line of sight, 
is by no means favourable. The most plausible way of ac- 
counting for the apparent figure of these nebulae is, therefore, 
to admit; that the expansion of the nebulosity consists indeed 
of a very narrow length, and not much depth. This form 
when ascribed to nebulous matter, is sufficiently uncommon 
for us to expect to see many nebulae of the figure of extended: 
rays. 
* See I. 33, 206. III. 254- IV. 72* V. 20 
