Biographical Memoir of Sir William HerschcL £15 
remotest stars in that particular part of the heavens. Hence he 
was able to deduce the probable form of the Milky Way, and 
the probable situation of the solar system within it. These in- 
teresting views are recorded in two Memoirs, u On the Construc- 
tion of the Heavens ,” published in the Philosophical Transac- 
tions for 1784 and 1785. 
In the year 1786, Dr Herschel presented to the Royal So- 
ciety “ A Catalogue of 1000 new Nebulae and Clusters of Stars f 
which he had observed with his £0 feet reflector since 1783, 
and this was followed in 1789 by “ A Catalogue of a second 1000 
new Nebulae and Clusters <f Stars , with a few introductory 
remarks on the Construction of the Heavens Having shewn 
that clusters of stars and these round nebulae, to the amount of 
2300, are either of a spherical form, or are more condensed, or 
brighter in the middle, Dr Herschel considers that this arrange- 
ment is owing to a central power residing in the brightest por- 
tion. The clusters which have the most perfect spherical figures, 
he supposes to have been longest exposed to the action of these 
forces, and he conceives that we may judge of the relative age 
and maturity of a sidereal system, from the disposition of its com- 
ponent parts, and that the same reasoning may be extended to 
nebulae, the degrees of brightness being supposed equivalent to 
the different accumulation of the stars in clusters. A cluster 
or nebula which is gradually more compressed and bright to- 
wards the middle, may be in the perfection of its growth, while 
others such as those called Planetary, where the compression is 
more equal, may be regarded as very aged, and approaching to 
a period of change or dissolution. 
These speculations, ingenious and sublime as they are, ex- 
cited less notice than the positive and important discoveries by 
which they were followed. Dr Herschel had now introduced 
the method of observing which he calls the Front View , which 
consists in laying aside the small speculum, and observing 
directly with an eye-glass the image formed by the large spe- 
culum. By this means he gained all the light (amounting 
nearly to one-half,) which was lost by reflexion, and he was 
particularly struck with the brightness and the facility with 
which he saw nebulae that he had observed in former surveys. He 
then thought of examining the Georgium Sidus by that new 
