relating to the Construction of the Heavens . 313 
whose appearance a considerable degree of solidity may be 
Inferred.* 
IV. 55 is 44 A pretty bright round nebula, almost of an even 
44 light throughout approaching to a planetar}/- appearance, 
44 but ill defined, and a little fainter on the edges ; it is about 
44 or 1 minute in diameter.” Fig. 34. 
IV. 37 is 44 A very bright planetary disk of about 35" in 
44 diameter, but ill denned on the edges ; the center of it is 
44 rather more luminous than the rest, and with long attention 
44 a very bright well defined round center becomes visible.” 
Fig- 35- 
In these nebulae we have three different indications of the 
compression of the nebulous matter of which they are com- 
posed: their figure, their light, and the small compass into 
which it is reduced. The round figure is a proof that the 
nebulous mass is collected into a globular form, which cannot 
have been effected without a certain degree of condensation. 
Their planetary appearance shews that we only see a su- 
perficial lustre such as opaque bodies exibit, and which could 
not happen if the nebulous matter had no other quality than 
that of shining, or had so little solidity as to be perfectly 
transparent. That there is a certain maximum of brightness 
occasioned by condensation, is to be inferred from the different 
degrees of light of round nebulas that are in a much less ad- 
vanced state of compression ; for these are gradually more 
bright towards the center; which proves that brightness keeps 
up with condensation till the increase of it brings on a con- 
* See Four nebulas of a planetary appearance. IV, 55, 60, 68, 78. 
Three planetary disks with a bright central point. II. 268. IV. 37, 73. 
S S 3 
